jee main practice

7
MATHEMATICS TODAY | APRIL ‘15  31 1. Te number o common divisors o 10800 and 9000 is (a) 49 (b) 36 (c) 25 (d) 64 2.  A, B are two students in a group o n students. I the number o ways o assigning the n students to a line o n single rooms such that A and B are not in adjacent rooms is 3600, then n = (a) 10 (b) 8 (c) 7 (d) 9 3. Te sides  AB, BC, CA o a triangle  ABC  have n, n + 1, n + 2 ( n N , n 3) interior points respectively on them. I the number o triangles ormed by any three o these (3n + 3) points is 205, then n = (a) 7 (b) 6 (c) 4 (d) 3 4. I the tangent and normal o a rectangular hyperbola xy  = 4 cut ointercepts a 1 , a 2  on x -axis and b 1 , b 2 on y -axis respectively, then a 1 a 2  + b 1 b 2  = (a) c 2  (b) 2c 2  (c) 0 (d) 4c 2  5. Te length o common tangent o hyperbolas x y 2 2 4 3 1 =  and x y 2 2 3 4 1 0 + =  is (a) 2 7  (b) 7 2  (c) 7 (d) 7 2  6. tan cot    +      ∫ ∫ 1 0 2 1 0 2 x dx x dx  = (where [.] represents the greatest integer unction ) (a) 1 – cot 2 (b) 1 + cot 2 (c) 2(1 + cot 2) (d) 2(1 – cot 2) 7. I 4 6 9 4 9 4 2 e e e e dx Ax B e C  x x x x x = + +  ∫ ln( ) ,  then  A+ B = (a) 9 35  (b) 35 36  (c) 9 36  (d) None 8. I  f x x x dx b a  f x c ( ) sin cos ( ) ln ( ) , = +  ∫ 1 2 2 2  then  f (x ) can be (a) 1 2 2 2 2 a x b x  cos sin +  (b) 1 2 2 2 2 b x a x  cos sin +  (c) a 2 cos 2 x  + b 2  s in 2 x  (d) b 2  c os 2 x  + a 2  s in 2 x  9. I sin 2 x  – 2sinx  – 1 = 0 has exactly our dierent solutions in x   [0, np], then minimum value o n’ can be (n  N ) (a) 4 (b) 3 (c) 2 (d) 1 10. P  ( a, b) is a point in rst quadrant. I two cir cles which pass through ‘P ’ and touch both the coordinate axes cut at right angles, then (a) a 2  6ab + b 2  = 0 (b) a 2  + 2ab b 2  = 0 (c) a 2  4ab + b 2  = 0 (d) a 2  8ab + b 2  = 0 11.  AB is diameter o a semi circle K , C  is an arbitrary point on the semicircle (other than  A or B ) and ‘ S’ is the centre o the circle inscribed in D  ABC , then measure o (a) angle  ASB changes as C  moves on K (b) angle  ASB is the same or all positions o C ’ but it cannot be determined without knowing the radius. (c) angle  ASB = 135° or all positions o C (d) angle  ASB = 150° or all positions o C Practice Paper 2015  JEE Main Exam on 4 th April * ALOK KUMAR, B.Tech, IIT Kanpur * Alok Kumar is a winner of INDIAN NATIONAL MATHEMATICS OLYMPIAD (INMO-91). He trains IIT and Olympiad aspirants.

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8/17/2019 Jee Main Practice

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MATHEMATICS TODAY 

| APRIL ‘15   31

1. Te number o common divisors o 10800 and

9000 is

(a) 49 (b) 36 (c) 25 (d) 64

2.  A, B are two students in a group o n students. I

the number o ways o assigning the n students to a line

o n single rooms such that A and B are not in adjacent

rooms is 3600, then n =(a) 10 (b) 8 (c) 7 (d) 9

3. Te sides  AB, BC, CA  o a triangle  ABC   have n,

n + 1, n + 2 (n ∈ N , n ≥ 3) interior points respectively on

them. I the number o triangles ormed by any three o

these (3n + 3) points is 205, then n =

(a) 7 (b) 6 (c) 4 (d) 3

4. I the tangent and normal o a rectangular

hyperbola xy  = 4 cut off intercepts a1, a2  on x -axis and

b1, b2 on y -axis respectively, then a1a2 + b1b2 =

(a) c2  (b) 2c2  (c) 0 (d) 4c2 

5. Te length o common tangent o hyperbolas

x y 2 2

4 31− =   and

x y 2 2

3 41 0− + =   is

(a) 27

  (b) 7 2   (c) 7 (d) 72

 

6. tan cot− −

 

  +  

   ∫ ∫ 

1

0

21

0

2

x dx x dx  =

(where [.] represents the greatest integer unction )

(a) 1 – cot 2 (b) 1 + cot 2

(c) 2(1 + cot 2) (d) 2(1 – cot 2)

7. I4 6

9 4

9 42e e

e e

dx Ax B e C  x x 

x x x −

−= + − +

− ∫  ln( ) ,   then

 A+ B =

(a)−9

35  (b)

35

36  (c)

9

36  (d) None

8. I  f x x x dx b a

 f x c( )sin cos( )

ln ( ) ,=−

+ ∫ 1

2 2 2 then

 f (x ) can be

(a)1

2 2 2 2a x b x  cos sin+ (b)

12 2 2 2b x a x  cos sin+

 

(c) a2cos2x  + b2 sin2x   (d) b2 cos2x  + a2 sin2x  

9. I sin2x  – 2sinx  – 1 = 0 has exactly our different

solutions in x  ∈ [0, np], then minimum value o ‘n’ can

be (n ∈ N )(a) 4 (b) 3 (c) 2 (d) 1

10. P  (a, b) is a point in first quadrant. I two circles

which pass through ‘P ’ and touch both the coordinate

axes cut at right angles, then

(a) a2 – 6ab + b2 = 0 (b) a2 + 2ab – b2 = 0

(c) a2 – 4ab + b2 = 0 (d) a2 – 8ab + b2 = 0

11.  AB is diameter o a semi circle K , C  is an arbitrarypoint on the semicircle (other than A orB ) and ‘S’ is the

centre o the circle inscribed in D ABC , then measure o (a) angle ASB changes as C  moves on K  (b) angle  ASB is the same or all positions o ‘C ’ but

it cannot be determined without knowing theradius.

(c) angle ASB = 135° or all positions o C (d) angle ASB = 150° or all positions o C 

Practice Paper 2015

JEE Main

Exam on

4th April

* ALOK KUMAR, B.Tech, IIT Kanpur 

* Alok Kumar is a winner of INDIAN NATIONAL MATHEMATICS OLYMPIAD (INMO-91).

He trains IIT and Olympiad aspirants.

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MATHEMATICS TODAY 

| APRIL ‘1532

12.x 

x dx 

7

2 51( )−= ∫   

(a)x 

x C 

8

2 41( )−+   (b)

x C 

8

2 84 1( )−+  

(c)x 

x C 

8

2 48 1( )−+   (d)

x C 

8

2 44 1( )−+  

13. I  f x x x x x ( ) ( ) ( )( )= + + + + + +1 1 1 1 2 4   then

 f x dx ( )   = ∫ 0

100

 

(a) 5010 (b) 5050(c) 5100 (d) 5000

14. Degree o differential equation o the curve

 y a e

a=   −

   

−1 ,  where ‘a’ being the parameter is

(a) 1 (b) 2(c) 3 (d) not defined

15. I solution o  y d y 

dx 

dy 

dx x 

2

2

2

   

  =   is given by

 y x 

C x C 23

1 2= + +l

,   then l =

(a) 2 (b) 3(c) 5 (d) 4

16. I x  ∈ R, then the maximum value o

 y a x x x b= − + +2 2 2( )( )  is

(a) a2 + b2  (b) a2 – b2 

(c) a2 + 2b2  (d) none o these

17. Te number o three digit numbers o the orm xyz

such that x < y  and z  ≤  y  is(a) 176 (b) 278(c) 276 (d) 240

18. I (1.5)30 = k, then the value o ( . ) ,1 52

29n

n=∑  is

(a) 2k – 3 (b) k + 1(c) 2k + 7 (d) 2k – 9/2

19. Te set o values o x  or which

1

1

1

1

1

1+   −   −x  x  x , ,   are in A.P. is given by

(a) (–∞, ∞) (b) (1, ∞)(c) [0, ∞) – {1} (d) [0, ∞)

20. Te ratio o nth  terms o two A.P.’s is

(14n – 6) : (8n + 23). Ten the ratio o their sum o first

m terms is

(a)4 4

7 24

m

m

++

  (b)7 1

4 24

m

m

−+  

(c)7 1

4 24

m

m

−−   (d)

7 1

4 27

m

m

++  

21. I  A =  −

1 1

1 1,  then A16 =

(a)0 256

256 0

  (b)

256 0

0 256

 

(c)−

16 0

0 16  (d)

0 16

16 0

 

22. angents are drawn to the circle x 2

 +  y 2

 = 12 at thepoint where it is met by the circle x 2 + y 2 – 5x  + 3 y  – 2 = 0.

Te point o intersection o these tangents is

(a)18

56,

   

  (b) 618

5, −

 

   

(c) (4, –5) (d) (5, –3)

23. Let a, b and c be positive real numbers. Ten the

ollowing system o equations in x, y  and z  

a

 y 

b

c

a

 y 

b

c

a

 y 

b

c

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

21 1 1+ − = − + = − + + =, ,

has(a) no solution (b) unique solution(c) infinitely many solutions(d) finitely many solutions

24. I the line lx + my + n = 0 cuts the ellipsex 

a

 y 

b

2

2

2

21+ =  

in points whose eccentric angles differ byp2

,   then

a l b m

n

2 2 2 2

2

+=  

(a) 1 (b) 2 (c) 4 (d) 3/2

25. I F x n x x x  n

n n( ) ( ),/ /( )= − >→∞

+lim 2 1 1 1 0  then

xF x dx  ( ) ∫  is equal to

(a)x 

x c2

2+ +ln   (b) − + + + +

x x x 

x c

2 2

4 2ln  

(c)x 

x x c3

3+ +ln   (d)

x x 

x c

2 2

2 4. ln   − +  

26. Te number o zero’s at the end o 60! is(a) 14 (b) 15 (c) 16 (d) 10

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MATHEMATICS TODAY 

| APRIL ‘15   33

27. Te number ormed by last two digits o the

number (17)256 is(a) 81 (b) 82 (c) 91 (d) 93

28. An ellipse o eccentricity2 2

3  is inscribed in a

circle and a point with in the circle is chosen at random.

Te probability that this point lies outside the ellipse, is(a) 1/9 (b) 4/9 (c) 1/3 (d) 2/3

29. In the binomial distribution 3

5

2

5

5

+

   

 i variance

is V  and standard deviation S, then(a) S = V 2  (b) 4(S – V 2) = V 2 (c) 5(V 2 – S2) = S2  (d) none o these

30. I C r   stands or nC r , then the sum o the series

(where n is an even positive integer)

22 2 2 3 1 10

212

22 2

n n

nC C C n C  n

n

 

 

 

 

  − + + + − +! !

![ ..... ( ) ( ) ]

is equal to(a) 0 (b) (–1)n/2(n + 1)(c) (–1)n(n + 2) (d) None o these

31. Te coefficient o x 3l   + 2, m  the expansion o

(a + x )l (b + x )l + 1 (c + x )l  + 2  (l  is a positive integer) is(a) l (a + b + c) (b) l (a + b + c) + b + 2c(c) l (a + b + c) + a + 2b + 3c (d) None o above

32. I n is a natural number , then nn – nC 1(n – 1)n +nC 2(n – 2)n – nC 3(n – 3)n + ....... must be equal to(a) n! (b) (n!)n  (c) nn! (d) 0

33. Tree normals are drawn to the curve y 2 = x   rom

a point (c, 0). Out o three, one is always on x -axis. I

two other normals are perpendicular to each other,

then the value o c is(a) 3/4 (b) 1/2 (c) 3/2 (d) 2

34. I area o triangle ormed by tangents rom the

point (x 1

, y 1

) to the parabola y 2 = 4ax   and their chord

o contact is

(a)( ) / y ax 

a

12

13 2

2

4

2

−  (b)

( ) / y ax 

a

12

13 3

2

4−

(c) ( ) / y ax 

a12

13 24

2

−   (d) none o these

35. A normal to the parabola y 2 = 4ax  with slope ‘m’

touches the rectangular hyperbola x 2 – y 2 = a2 i,(a) m6 + 4m4 – 3m2 + 1 = 0(b) m6 – 4m4 + 3m2 – 1 = 0

(c) m6

 + 4m4

 + 3m2

 + 1 = 0(d) m6 – 4m4 – 3m2 + 1 = 0

36. Te number o ways o selecting 10 balls out o an

unlimited number o white, red, blue and green balls is(a) 270 (b) 84 (c) 286 (d) 86

37. A five digit number divisible by 3 is to be ormed

using the numerals 0, 1, 2, 3, 4 and 5 without repetition.

Te total number o ways in which this can be done is

(a) 216 (b) 240 (c) 600 (d) 3125

38. Te number o ways in which 7 persons can be

seated at a round table i two particular persons are not

to sit together is(a) 120 (b) 480(c) 600 (d) 720

39.  A is a set containing ‘n’ elements. A subset P  o  A 

is chosen. Te set  A is reconstructed by replacing the

elements o P . A subset Q  o  A  is again chosen. Te

number o ways o choosing P   and Q, so that P   ∩  Q 

contain exactly two elements is(a) 9 nC 2  (b) 3n – nC 2 (c) 2 nC n  (d) nC 2 × 3n – 2 

40. Te number o +ve integer satisying the inequalityn+1C n–2 – n+1C n–1 ≤ 100 is(a) nine (b) eight(c) five (d) None o these

41. Te solution o the differential equation

dy 

dx 

 y y dx = +

 ∫ 0

1

,  given that  y  = 1, when x  = 0 is

(a) 2 1

3

e e

e

x − +−

  (b)2 1

3

e e

e

x + ++  

(c)2 1

3

e e

e

x + +−

  (d)e e

e

x − +−2 1

42. Te polynomial unction  f (x ) o degree 6, which

satisfies lim

/

x  f x 

x e

→+

  ( )

  

  =0 3

121  and has local maximum

at x  = 1 and local minimum at x  = 0 and x  = 2 is

(a)  f x x x x ( ) = − +2 122

34 5 6

 

(b)  f x x x x ( ) = − +212

524 5 6

 

(c)  f x x x x ( ) = − +212

5

2

34 5 6

 

(d)  f x x x x ( ) = + −4 5 6125 23  

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MATHEMATICS TODAY 

| APRIL ‘1534

43. I  f x x xy x y f y dy ( ) ( ) ( ) ,= + + ∫  2 2

0

1

 then f (x ) is

(a)180

119

80

1192x x +   (b)

180

119

80

1192x x −

(c) − +180119

80119

2x x    (d) − +    180119

80119

2x x   

44. A periodic unction with period ‘1’ is integrable

over any finite interval. Also or two real numbers a, b 

and or two unequal non-zero positive integers m & n

 f x dx f x dx 

b

b m

a

a n

( ) ( )=++

 ∫  ∫  , then the value o  f x dx 

m

n

( ) ∫   is

(a) 0 (b) 1/2 (c) 2 (d) mn

45. I  y arc x 

dy dx 

=  

   

cos cos

cos,3

3then  is

(a) 3

4 2cos cosx x +  (b)

2

4 2cos cosx x +

(c)6

4 2cos cosx x +  (d) 6

4 2cos cosx x − 

SOLUTIONS

1. (b) : 9000 = 23 × 53 × 32 and 10800 = 24 × 33 × 52.

A common divisor is o the orm 2 3 51 2 3a a   a ,   where0 ≤ a1 ≤ 3, 0 ≤ a2 ≤ 2, 0 ≤ a3 ≤ 2. Hence, number ocommon divisors is (3 + 1)(2 + 1)(2 + 1) = 36.

2. (c) : reating 2 adjacent rooms as a single unit,number o ways o assigning n rooms to n students is2(n – 1)! and total number o ways o assigning roomsis n!. Hence n! – 2(n – 1)! = 3600 (given)i.e., (n – 2)(n – 1)! = 5(6!) ⇒  n = 7

3. (d) : Tere are totally (3n + 3) points and i noneo them are collinear, we can orm (3n+3)C 3 triangles.

But by joining any 3 on  AB (or AC  or BC ), we do notget any triangle, hence given(3n+3)C 3 – (nC 3 + (n+1)C 3 + (n+2)C 3) = 205 ⇒ n = 3

4. (c) : angent is perpendicular to normal at any

point and slope o a line = −intercept on -axis

intercept on -axis

 y 

5. (b) :  y mx m= + −4 32  is a tangent to 1st  hyperbolaand it becomes a common tangent i m = ± 1. Hencecommon tangents are y  = ±x  + 1. Te points o contact

are (–4, –3) and (3, 4), hence length o common tangentis 7 2  

6. (c) : [tan ]; tan

;− =

  ≤ <≥

1 0 1

1x 

 0

  tan1 

[cot ]; cot

;− =

  ≤ ≤

1 1 1

0x 

 0

  >cot1

 1 1 2 1 1 2 1 2

0

1

1

2dx dx  + = − + = + ∫  ∫ 

cot

tan

tan cot ( cot )

7. (b) : 4 6 9 4 9 4e e A e e Bd 

dx e ex x x x x x  − = − + −− − −( ) ( )

8. (b) :  f x x x b a

 f x 

 f x ( ) sin cos

( )

( )

( )  =

1

2 2 2−

′ 

⇒ 2 22 22

b x x a x x   f x 

 f x sin cos sin cos

( )

( ( ))− =

  ′

Integrating both sides w.r.t ‘x ’, we get

− − =  −

b x a x   f x 

2 2 2 2 1cos sin

( )

⇒  f x a x b x  

( )sin cos

=+

12 2 2 2

9. (a) : (sin ) sinx x − = ⇒ − = ±1 2 1 22

⇒ sin x  = −1 2  (since sinx  is not greater than 1)

2 solutions in [0, 2p] and two more in [2p, 4p]

10. (c) : Let the circle be x y cx c y c2 2 2 2 0+ − − + =  

Since it passes through (a, b), we have

  c c a b a b− + + + =2 02 2( )

⇒  c c a b c c a b1 2 1 22 22+ = + = +( );  

  Te circles are orthogonal

\  4 1 2 1 2c c c c= + ,  ⇒ a2 – 4ab + b2 = 0

11. (c) : ∠ = ° ∠ = °+ = °C ASB C 90 902

135,  

12. (c) :

1

11

11

3

2

5 2x 

dx x 

   

− = ∫  , Let  

⇒ − = −−

   =

   

+ =−

+−  −

 ∫ 1

2

1

2 4

1

8  1

18 1

54

2

4

8

2 4t dt 

  t 

C   x 

( )

13. (c) : f (x ) = (x  + 1), x  ∈ [0, 100]

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MATHEMATICS TODAY 

| APRIL ‘15   35

14. (d) : D.E.  ormed afer eliminating a is

 y x 

dy 

dx 

dy 

dx = −

   

   

log

1

Hence degree is not defined.

15. (b) : d dx 

 y  dy dx 

x          =  

⇒ = + \ = + +

\ = + + \ =

 y dy 

dx 

x P 

 y x Px q

 y x 

C x C 

2 23

23

1 2

2 2

1

6

33l

 

16. (a) : Let x x b t  + + =22  

⇒  x b x b

2 22

+ − =

⇒  2 22 22 2

x b t b

t x t 

b

t + = + = −; .

\ − + +( )( )2 2 2 2a x x x b 

= − +

 

    = − +

= + − − ≤ +

2 22

2 2

2 2 2 2 2

a t b

t t at t b

a b t a a b( ) ( )

17. (c) : I zero is included, it will be at z,  then no.o three digit numbers = 9C 2. I zero is excluded,

then

x y z C  

x z y C  

x y z C  

, , !all di  ⇒ ×

= < ⇒

< = ⇒

93

92

92

2

\  otal number o ways = 276

18. (d): S kr 

= − − =  −

  − = −

=

∑ ( . ) .( . )

.

.1 5 1 1 51 5 1

1 5 1

2 5 29

20

29 30

 

19. (c) :1

1

1

1

2

1+  +

−  =

−x x  x  is true or all x  and it

is defined or x  ≥ 0, x  ≠ 1

20. (d) :T 

a n d 

a n d 

a n d 

a n d 

nn

n′  =

  + −+ −

  =  + −

+ −  =

  −1 1

2 2

1 1

2 2

1

1

2 2 2

2 2 2

14 6

8

( )

( )

( )

( ) nn+23

Put 2n – 2 = m –1 in both sides,

⇒2 1

2 1

7 1

4 271 12 2

a m d 

a m d 

m

m

+ −+ −   =

  ++

( )

( )  

21. (b) :  A A A2 4 80 2

2 0

4 0

0 4

16 0

0 16=

  −

  =

  −−

  =

, , ;

 

 A16 256 0

0 256=

 

22. (b) : Common chord is 5x – 3 y – 10 = 0. Te chordo contact o required point w.r.t circle x 2 + y 2 = 12 isalso the common chord o given two circles. Chord ocontact o P (x 1,  y 1) is xx 1 + yy 1 – 12 = 0

⇒x y 1 1

5 3

12

10=

−  =

 −−

 

23. (d) : Letx 

a X 

 y 

bY 

cZ 

2

2

2

2

2

2= = =, ,  ⇒  X + Y – Z  = 1,

 X  – Y + Z  = 1, – X + Y + Z  = 1. On solving X = Y = Z = 1⇒  x  = ± a,  y  = ± b, z  = ± c  ⇒  8 solutions

24. (b) : Let the point o intersection o the line andthe ellipse be (a cosq, bsinq) and

a bcos , sin .p

q  p

q2 2

+

   

  +

   

   

Since they lie on the

given line lx + my + n = 0,la cosq + mb sinq +n = 0 ⇒ la cosq +mb sinq = n and–la sinq + mb cosq + n = 0 ⇒ la sinq – mb cosq = n.Squaring and adding, we get

 

a l b m na l b m

n

2 2 2 2 22 2 2 2

22 2+ = ⇒

  +=

25. (d) : F x n x x  n

n n n( )

/ ( )= −

→∞

+( )   +lim 2 1 1

1

11

=−

 

+  ×

  +  =

→∞

+   +( )

limn

n n nx x 

n n

n n

n

1 1 1 1

2

1

1

1

1

/( ) /

( )

( )log

Hence,

xF x dx x x dx  x 

x x c( ) log log= = − + ∫ ∫ 2

2

2

1

4

26. (a) : Number o zero’s at the end o60! = exponent o 10 in 60!

= min {E2(60!), E5(60!)} = E5(60!) = 14

27. (a) : (17)256 = (289)128 = (300 – 11)128 

= 128C0(–11)128 + 100m, or some integer m

= 11128 + 100m = (10 + 1)128 + 100m 

= 128C 01128   + 128C 110 + 100m1  + 100m  or some

integer m1

= 1 + 1280 + 100k, (m + m1 = k)

= 1281 + 100kHence the required number is 81.

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MATHEMATICS TODAY 

| APRIL ‘1536

28. (d) : Let the radius o the circle be a, then the majoraxis o the inscribed ellipse is o length 2a.

Te required probability =  − −p p

p

a a e

a

2 2 2

2

1

 = − −1 1 2e   = − − =1 1

8

9

2

3(area o ellipse = = ⋅ −p pab a a e1 2 ,

‘e’ being eccentricity)

29. (c) : V npq= = =53

5

2

5

6

5. .  

S =6

5 and 5 5

36

25

6

52 2( )V S− = −

 

 

 = −

 

 5

36

25

30

25  = =

6

5

2S 

30. (d) : Let us test the choice or n = 2For n = 2, the series

= − ⋅ ⋅ = −2 1 1

240

22

( !)( !)

![ ]2 2

12 2 2C 2 C +3 C

 Now observe the table

Choice Value at n = 2

(a) 0

(b) –3

(c) 4Since none o the choices become – 4, the correctchoice must be (d).

31. (b) : Given expression = (x  + a) (x  + a) . . . l  times(x + b) (x + b) ………( l  + 1) times × (x + c) (x + c). . . ( l  + 2) times

 = + + + +x al b l c l x  l  l 3 3

1+3 + 2+ + 2[ ( ) ( )] ...

⇒  co-efficient o x 3l  + 2 is (a + b + c)l + b + 2c Choice (b) is correct.

32. (a) : nn

 –n

C 1(n – 1)n

 +n

C 2(n – 2)n

 + ....Number o ways o districting n objects in n districtcells such that no cell remains empty = n!

33. (a) : Normal at (at 2, 2at ) is y + tx  = 2at  + at 3 a =

   

1

I this passes through (c, 0)

We have, ct  = 2at  + at 3 =t t 

2+

4

3

 

⇒  t  = 0 or t 2 = 4c – 2I t   = 0, the point at which the normal is drawn is

(0, 0). I t  ≠ 0, then the two values o t  represents slopeo normals through (c, 0)

I these normals are perpendicular, then

( )( ) ( )( )− − = − ⇒ = − ⇒ − − − = −t t t t c c1 2 1 21 1 4 2 4 2 1

⇒ c =3

4

34. (c) : Let A(x 1, y 1) be any point outside the parabola

and B(a, b), C (a′, b′) be the points o contact o tangentsrom point A. Eq. o chord BC ,  yy 1 = 2a(x  + x 1)Lengths o ^  AL rom A to BC 

 ALa x x y y  

 y a

 y ax 

 y a=

  −=

  −2 +

+ 4

4

+ 4

1 1

2

12

12 2

( )1 1

12

1

Area o =1

2D ABC AL BC ×

We get,41

21( ) / y ax 

a

− 3 2

2

35. (c) : Equation o the normal to the parabola y 2 = 4ax  with slope m is y  = mx  – 2am – am3. It touchesthe rectangular hyperbola x 2 – y 2 = a2 i(–2am – am3)2 = a2(m2 – 1)

⇒  a2m2(2 + m2)2 = a2(m2 – 1)

⇒  m2(m4 + 4m2 + 4) = m2 – 1

⇒  m6 + 4m4 + 3m2 + 1 = 0

36. (c) : Let x 1, x 2, x 3 and x 4, be the no. o white, red,blue and green balls that are selected. Ten x 1 + x 2 +x 3 + x 4 = 10. Te required no. o ways

= coefficient o y 10 in (1 +  y + y 2 +  y 3 + ….)4= coefficient o y 10 in (1 –  y )–4

= coefficient o y 10 in (1 + 4C 1  y  + 5C 2  y 2 + 6C 3  y 3 + ...+ 13C 10 y 10....)

= 133

13 12 11

2 3286C   =

  × ××

  =

37. (a) : Te sum o the numerals 0, 1, 2, 3, 4 and 5is 15. We know that a five digit number is divisibleby 3 i and only i the sum o its digits is divisible by3. Tereore, we should not use either 0 or 3 whileorming the five digit number. I we do not use 0, then

the remaining digits can be arranged in 5P 5 = 5! = 120ways. I we do not use 3, then the remaining digitscan be arranged in 5P 5 – 4P 4 = 5! – 4! = 120 – 24 = 96ways to obtain a five digit number. Tus, the total no.o such 5 digit numbers is 120 + 96 = 216.

38. (b) : 6! – 2! 5! = 480

39. (d) : A = {a1, a2, …., an}

  (i) ai ∈ P , ai ∈ Q

  (ii) ai ∈ P , ai ∉ Q

  (iii) ai

 ∉ P , ai

 ∈ Q

  (iv) ai ∉ P , ai ∉ Q

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MATHEMATICS TODAY 

| APRIL ‘15   37

P  ∩ Q contains exactly two elements, taking 2 elementsin (i) and (n – 2) elements in (ii) or (iii) or (iv)No. o ways = nC 2 × 3n–2

40. (b) : n+1C n–2 – n+1C n–1 ≤ 100

⇒ − ≤ ⇒ + − −+

≤+ +n n

C C n n n  n n1

31

2   100  1

6

1 1  1

2

100( ) ( )  ( )

⇒ + − − ≤ ⇒ + − ≤1

61 1 3 100 1 4 600n n n n n n( ){ } ( )( )

It is true or n = 2, 3, 4, 5, 6, 7, 8, 9

41. (a) : Let  y f x  f x 

 f x = ⇒

  ′′

′  =( )

( )

( )1  

⇒ = + f x Ce Dx ( ) ( )  

At x  = 0,  y f x C e

De

= = ⇒ =−

  = −−

( ) ,12

31

2

42. (c) :  f x ax bx cx ( ) = + +4 5 6  

a f x x bx cx  =   ′   = + +2 8 5 63 2, ( ) ( )

b c= − =12 5 2 3/ , /

43. (a) :  A y f y dy B y f y dy = = ∫ ∫ 2

0

1

0

1

( ) , ( )  

 f x x Ax Bx ( ) = + + 2

 ⇒ = = A B

61

119

80

119,

 

44. (a) : f (1 + x ) = f (x )

\ = =

 ∫  ∫  ∫ 

+

 f x dx f x dx n f x dx n

a

a n

( ) ( ) ( )

0

1

0  

similarly only m f x dx  ( )

0

1

 ∫   

⇒ ( ) ( )n m f x dx  − = ∫ 0

1

0   ⇒ = ∫  f x dx ( )

0

1

0

  f x dx f m x dx f x dx 

m

n n m n m

( ) ( ) ( ) ∫ ∫ ∫ = + =− −

0 0

 

= − = ∫ ( ) ( )n m f x dx  

0

10

45. (c) : coscos

cos

23

3 y 

x =  

⇒dy 

dx 

 y =

3 2sec

cos

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