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  • Meaning of Proof

    Methods of Proof

    Mathematical Proof

    4/7/2016 1 Dr. Priya Mathew SJCE Mysore Mathematics Education

  • Introduction  Proposition: Proposition or a Statement is a grammatically

    correct declarative sentence which makes sense, which is either true or false, but not both.

     Example: Sum of 2 and 5 is 7 ( proposition which is true)  Sum of 2 and 6 is 7 - (Proposition which is false)  Is 7 the sum of 2 and 5? -(not a proposition , it is not a declarative sentence )  Sum of 2 and 5 is green. ( not a proposition as the sentence does not make any sense) • Majula is a good student ( sentence is vague and does not have a definite truth value)  The method of establishing the truth of a proposition is

    known as proof

    4/7/2016 2 Dr. Priya Mathew SJCE Mysore Mathematics Education

  •  Method of establishing the logical validity of the conclusion of a theorem, as a consequence of the premise, axioms, definitions and already established theorems of the mathematical system is known proof.

     Proof is a deductive argument for a mathematical statement rather than inductive or empirical argument.

     In the argument previously established statements or theorems can be used

     A process which can establish the truth of a mathematical statement based purely on logical arguments

    4/7/2016 3 Dr. Priya Mathew SJCE Mysore Mathematics Education

  •  The word comes from the Latin Word

    Probare’ means to test’  Probe (English) : to check in detail

     Probar (Spanish) : to taste, smell, to touch or

    to test

     Provare (Italian) : to try

     Probieren (German) : to try

    4/7/2016 4 Dr. Priya Mathew SJCE Mysore Mathematics Education

  •  A statement that is proved is often called as a

    theorem.

     Once a theorem is proved it can be used as

    the basis to prove further statements.

     A proof must demonstrate that a statement

    is always true, rather than enumerating many

    conformity cases.

    4/7/2016 5 Dr. Priya Mathew SJCE Mysore Mathematics Education

  •  Verify that : 1. The product of two even number is even 2. Sum of any even number is even 3. Sum of the interior angles of a triangle is 180  In verification, we cannot physically check the

    products of all possible pairs of even numbers  It may help us to make statement we believe is

    true.  We cannot be sure that it is true for all cases.  Verification can often be misleading

    4/7/2016 6 Dr. Priya Mathew SJCE Mysore Mathematics Education

  •  To establish that a mathematical statement is

    false, it is enough to produce a single

    counter example

     7+5 = 12 is a counter example to the

    statement that the sum of two odd

    numbers is odd.

     there is no need to establish the validity of a

    mathematical statement by checking or

    verifying it for thousands of cases.

    4/7/2016 7 Dr. Priya Mathew SJCE Mysore Mathematics Education

  • a) Direct Proof

    b) Indirect Proof

    c) Proof by counter examples

    d) Proof by induction

    4/7/2016 8 Dr. Priya Mathew SJCE Mysore Mathematics Education

  •  It is the most familiar proof  It is to prove statements of the form if P then

    Q . i.e PQ.  This method of proof is to take an original

    statement P , which we assume to be true and use it to show directly that another statement Q is true.

     Example : If a, b are two odd natural numbers, then the product of a and b is also an odd number

    4/7/2016 9 Dr. Priya Mathew SJCE Mysore Mathematics Education

  • Proof: Any odd number can be written as 2n+1 , where n

    is any natural number.  Let a = 2n+1 and b = 2m+1  So a.b = (2n+1) (2m+1)  = 4mn + 2n+ 2m+1  = 2 (2mn + m +n) +1  = 2(p +1) where, p = 2mn + m +n Therefore, 2p+1 is an odd number.  a.b is an odd number Eg. 2: Prove that product of two even natural

    numbers is even 4/7/2016 10 Dr. Priya Mathew SJCE Mysore Mathematics Education

  •  It is synonyms with proof by contradiction

     In this, a statement to be proved is assumed

    false for the sake of reasoning, and if the

    assumption leads to an impossibility or

    contradiction, then the statement assumed

    false in the beginning is proved to be true.

     The idea behind it is that if what you assumed

    creates a contradiction, the opposite of your

    initial assumption is the truth.

    4/7/2016 11 Dr. Priya Mathew SJCE Mysore Mathematics Education

  • 1. Proof by contradiction: It is used normally,

    when it is not straight forward as to how to

    proceed with the proof.

    We assume to the contrary that the conclusion

    is false and by logical arguments arrive at

    something which is absurd.

    4/7/2016 12 Dr. Priya Mathew SJCE Mysore Mathematics Education

  •  Prove that sum of any even number is even Proof: Let us assume + +6+8+…….+ n is an odd

    number, where n>0  + + + +…….+n is an odd number. 2[n(n+1)/2] is an odd number  n(n+1) is an odd number Which is a contradiction, since n(n+1) is always

    an even number. Hence the proof.

    4/7/2016 13 Dr. Priya Mathew SJCE Mysore Mathematics Education

  •  Assume that the conclusion is false

     Establish a contradiction : Establish that the

    assumption which was considered as false

    contradicts some previously existing

    theorem, definition, postulates, etc.

     State that the assumption must be false,

    thus the conclusion or the statement to be

    proved is true.

    4/7/2016 14 Dr. Priya Mathew SJCE Mysore Mathematics Education

  • 2. Proof by contra positive : It is based on the fact that a proposition p q is equivalent to its contra positive ≈q  ≈p

     We assume that the negation of the conclusion is true and in straight forward way show that the negation of the hypothesis is true.

     i.e if the conclusion is false, then the premises are false. But the premises are true, therefore, the conclusion must be true.

    4/7/2016 15 Dr. Priya Mathew SJCE Mysore Mathematics Education

  •  Theorem: If n2 is odd then n is odd

     Proof : its contrapositive is

     if n is not odd then n2 is not odd  To prove contrapositive assume that n is not odd

     Therefore n = m for some integer m  therefore n2 = (2m)2 = 4 m2 = 2(2 m2 ) is even

     Hence n2 is not odd

     Therefore, If n2 is odd then n is odd

    4/7/2016 16 Dr. Priya Mathew SJCE Mysore Mathematics Education

  •  If we have a statement involving a universal quantifier and to prove that the statement is true, we have to show that the statement is true for every element in the universal set.

     To show that such a statement is false, it is sufficient to show that there is a particular value in the universal set for which the statement is false.

     Exhibiting such value for which the statement is false is known as counter example.

     Method of giving such a counter example to show that such a statement is false is known as method of disproof or proof by counter example 4/7/2016 17 Dr. Priya Mathew SJCE Mysore Mathematics Education

  •  Statement : Every odd natural number is a prime

     To disprove this, it is sufficient to give example of one odd natural number which is not a prime.

     Consider a natural number 9 it is odd as 9 = 2x4 +1

     however it is not a prime since 9 = 3x3  So, 9 is an odd natural number which is not a

    prime.  Hence, the given statement is not true.

    4/7/2016 18 Dr. Priya Mathew SJCE Mysore Mathematics Education

  •  The sum of two odd numbers is odd

     Counter Example , 7+5 =12

    4/7/2016 19 Dr. Priya Mathew SJCE Mysore Mathematics Education

  •  Mathematical induction is a mathematical

    proof technique

     used to establish a given statement for all

    natural numbers, although it can be used to

    prove statements about any well-ordered set.

     It is a form of direct proof, and it is done in

    two steps.

    4/7/2016 20 Dr. Priya Mathew SJCE Mysore Mathematics Education

  •  The basis (base case): prove that the statement holds for the first natural number n. Usually, n = 0 or n = 1, ra

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