SAS Similarity

SAS Similarity : If in two triangles, one pair of corresponding sides are proportional and the included angles are equal then the two triangles are similar.

Given : Two triangles ABC and DEF such that ∠A = ∠D
AB        AC
---- = ------
DE         DF

Prove that : ΔABC ~ ΔDEF

Construction : Let P and Q be two points on DE and DF respectively such that DP = AB and DQ = AC. Join PQ.


Statements
Reasons
1) AB = DP ; ∠A = ∠D and AC = DQ 1) Given and by construction
2) ΔABC ≅ ΔDPQ 2) By SAS postulate
3) AB       AC
   ---- = ------
    DE       DF 		3) Given
4) DP       DQ
   ---- = ------
   DE        DF 		4) By substitution
5) PQ || EF 5) By converse of basic proportionality theorem
6) ∠DPQ = ∠E and ∠DQP = ∠F 6) Corresponding angles
7) ΔDPQ ~ ΔDEF 7) By AAA similarity
8) ΔABC ~ ΔDEF 8) From (2) and (7)

Examples

1) In the given figure, if QT / PR = QR / QS and ∠1 = ∠2.

Prove that ΔPQS ~ ΔTQR.

Given : QT / PR = QR / QS and ∠1 = ∠2

Prove that : ΔPQS ~ ΔTQR.

Statements
Reasons
1) QT / PR = QR / QS 1) Given
2) QT / QR = PR / QS 2) By alternendo
3) ∠1 = ∠2 3) Given
4) PR = PQ 4) Side opposite to equal angles are equal.
5) QT / QR = PQ / QS 5) By substitution from (2)
6) PQ / QT = QS / QR 6) By alternendo
7) ∠PQS = ∠TQR 7) Reflexive (common) in Δ PQS and ΔTQR
8) ΔPQS ~ Δ TQR 8) By SAS postulate


Criteria for Similarity

• AAA Similarity
• AA Similarity
• SSS Similarity
• SAS Similarity
• Practice on Similarity

From SAS Similarity to Criteria for Similarity of Triangles

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