Form 1 MathematicsChapter 11

Lesson requirement Textbook 1B Workbook 1B Notebook

Before lessons start Desks in good order! No rubbish around! No toilets!

Keep your folder at home Prepare for Final Exam

Missing HW Detention

SHW (I) 8th May (Wednesday)

SHW (II) 14th May (Tuesday)

OBQ 15th May (Wednesday)

CBQ 20th May (Monday)

Congruent figures (全等圖形 )

1. Figures having the same shape and size are called

congruent figures.

e.g. Figures X and Y below are congruent figures.

2. Two congruent figures can fit exactly on each other.

X Y

We learnt transformations in Chapter 9:

Translation Reflection Rotation Enlargement and Reduction

Which one will NOT change the shape?

Try Class Discussion on p.174

1. When a figure is translated, rotated or reflected,the

image produced is congruent to the original figure.

2. When a figure is reduced or enlarged, the image

produced will not be congruent to the original one.

Symbol “ ” means “is congruent to”When two triangles are congruent,

(i) their corresponding sides (對應邊 ) are equal,

(ii) their corresponding angles (對應角 ) are equal.

e.g. If △ABC △XYZ, then

AB = XY,

BC = YZ,

CA = ZX,

A = X,

B = Y,

C = Z.

A

B C

X

Y Z

Example 1:Name a pair of congruent triangles in the figure.

From the figure above,

we have △ABC △RQP.

Example 2:Given that △ABC △XYZ, find the unknowns x, b and y.

∵ The corresponding sides and corresponding angles

of two congruent triangles are equal,

∴ x = 5 cm, b = 6 cm, y = 50°

Example 3:Given that △PQS △RSQ,

find the unknowns s and r.

Since △PQS △RSQ,we have PQ = RSi.e. s = 8 cm

and RQS = PSQ = 70°

In △RSQ,45° + RQS + r = 180° 45° + 70° + r = 180°

115° + r = 180°

r = 65°

Page 176 of Textbook 1B Class Practice

Pages 177 – 178 of Textbook 1B Questions 4 – 17

Pages 74 – 75 of Workbook 1B Questions 2 – 5

There are four common conditions:

SSS: 3 Sides Equal

SAS: 2 Sides and Their Included Angle Equal

ASA : 2 Angles and 1 Side Equal(AAS)

RHS: 1 Right-angle, 1 Hypotenuses (斜邊 )

and 1 Side Equal

If AB = XY, BC = YZ and CA = ZX,

then △ABC △XYZ.

[Reference: SSS]

Example 1:Determine which pair of triangles in the following are

congruent.

The lengths of the three sides of (I) and (III) are 4, 6 and 8.

∴ (I) and (III) are congruent triangles. (SSS)

Example 2:

Write down a pair of congruent triangles and give reasons.

AB = CD (Given)

BD = DB (Common side)

DA = BC (Given)

△ABD △CDB (SSS)

If AB = XY, B = Y and BC = YZ,

then △ABC △XYZ.

[Reference: SAS]

Example 1:Determine which pair of triangles in the following are

congruent.

The lengths of the two sides of (I) and (III) are 5 and 6 and their included angles are both 40°.

∴ (I) and (III) are congruent triangles. (SAS)

Example 2:

Write down a pair of congruent triangles and give reasons.

CA = CE (Given)

CB = CD (Given)

ACB = ECD (Given)

△ACB △ECD (SAS)

Note: Must be SAS, not SSA!The abbreviation for this condition for congruent triangles is SAS, where the ‘A’ is written between the two ‘S’s to indicate an included angle. If we write SSA, then it means ‘two sides and a non-included angle’, but this is not a condition for congruent triangles. For example:

If A = X , AB = XY

and B = Y,

then △ABC △XYZ.

[Reference: ASA]

or

If A = X , B = Y

and BC = YZ,

then △ABC △XYZ.

[Reference: AAS]

Example 1:

The two angles of (I) and (IV) are 45° and 70° while their included sides are both 8.

∴ (I) and (IV) are congruent triangles. (ASA)

The two angles of (II) and (III) are 45° and 70° while the lengths of the sides opposite to 70° are both 8.

∴ (II) and (III) are congruent triangles. (AAS)

Determine which pair(s) of triangles in the following are congruent.

Example 2:Write down a pair of congruent triangles and give reasons.

BAD = CAD (Given)

AD = AD (Common side)

ADB = ADC (Given)

△ADB △ADC (ASA)

Example 3:Write down a pair of congruent triangles and give reasons.

ABD = ACD (Given)

BDA = CDA (Given)

DA = DA (Common side)

△BDA △CDA (AAS)

If C = Z = 90°, AB = XY and BC = YZ,

then △ABC △XYZ.

[Reference: RHS]

Example 1:

Determine which pair of triangles in the following are

congruent.

(I) and (III) are both right-angled triangles. Also, the hypotenuses and the sides of (I) and (III) are both 6 and 4 respectively.

∴ (I) and (III) are congruent triangles.

Example 2:In the figure, CAD and ACB are both right angles

and DC = BA. Determine whether △DAC and △BCA

are congruent and give reasons.

DAC = BCA = 90

DC = BA (Given)

AC = CA (Common side)

Yes, △DAC △BCA (RHS).

The table below summarizes all the conditions needed for two triangles to be congruent:

SSS SAS ASA AAS RHS

Page 185 of Textbook 1B Class Practice

Pages 186 – 187 of Textbook 1B Questions 1 – 17

Pages 76 – 79 of Workbook 1B Questions 1 – 5

Enjoy the world of Mathematics!

Ronald HUI