Side Angle Side Postulate

Side angle side postulate -> If two sides and the included angle of a triangle are congruent to two sides and the included angle of another triangle, then the two triangles are congruent by side angle side postulate.

AB ≅ DE, BC ≅ EF and ∠B ≅ ∠E
∴ ΔABC ≅ Δ DEF by SAS

Theorem : Angles opposite to two equal sides of a triangle are equal.

Given : Δ ABC in which AB = AC

Prove that : ∠C = ∠B


Construction : Draw the bisector AD of ∠A which meets BC in D.

Statements	Reasons
1) AB = AC	1) Given
2) AD is a bisector	2) By construction
3) ∠BAD = ∠CAD	3) By definition of angle bisector
4) AD = AD	4) Reflexive (common side)
5) ΔABD ≅ ΔACD	5) SAS Postulate
6) ∠B = ∠C	6) CPCTC

Examples :

1) O is the mid point of AB and CD. Prove that
i) ΔAOC ≅ ΔBOD ii) AC = BD and iii) AC || BD .

Given : O is the mid point of AB and CD.

Prove that : i) ΔAOC ≅ ΔBOD ii) AC = BD and iii) AC || BD .

Statements	Reasons
1) O is the mid point.	1) Given
2) AO = OB	2) By definition of mid point.
3) ∠AOC = ∠BOD	3) Vertically opposite angles
4) CO = OD	4) By definition of mid point.
5) ΔAOC ≅ ΔBOD	5) SAS postulate
6) AC = BD	6) CPCTC
7) ∠CAO = ∠DBO	7) CPCTC
8) AC || BD	8) If alternate interior angles are congruent then the lines are parallel.

2) If D is the mid point of the hypotenuse AC of a right triangle ABC, prove that BD = ½ AC.

Given : ΔABC in which ∠B = 90 ⁰ and D is the mid point of AC.

Prove that : BD = ½ AC.

Construction : Produce BD to E so that BD = DE. Join EC.

Statements	Reasons
1) AD = DC	1) Given
2) BD = DE	2) By construction
3) ∠ADB = ∠CDE	3) Vertically opposite angles
4) ΔADB ≅ ΔCDE	4) By SAS postulate
5) EC = AB and ∠CED = ∠ABD	5) CPCTC
6) CE || AB	6) If alternate interior angles are congruent then the lines are parallel
7) ∠ABC + ∠ECB = 180	7) Angles formed on the same side of transveral are supplementary.
8) 90 + ∠ECB = 180	8) Since ∠B = 90 given
9) ∠ECB = 180 -90 = 90	9) By subtraction property
10) AB = EC	10) From (5)
11) BC = CB	11) Reflexive (Common side)
12) ∠ABC = ∠ECB	12) Each 900
13) ΔABC ≅ ΔECB	13) SAS postulate
14) AC = BE	14) CPCTC
15) 1/2AC = 1/2BE ⇒ 1/2AC = BD	15) Multiply by 1/2 but 1/2BE = BD by mid point definition.

---

• Side Angle Side Postulate
• Side Side Side Postulate
• Angle Angle Side Postulate
• Angle Side Angle Postulate
• HL postulate (Hypotenuse – Leg OR RHS)

From side angle side postulate to Postulates of Congruent Triangle

Home Page