Theorems

Theorem 6.1 - Basic Proportionality Theorem (BPT)
Important

Theorem 6.2 - Converse of Basic Proportionality Theorem

Theorem 6.3

AA Similarity Criteria

Theorem 6.4 Important You are here

Theorem 6.5

Theorem 6.6 Important Deleted for CBSE Board 2025 Exams

Theorem 6.7 Important Deleted for CBSE Board 2025 Exams

Theorem 6.8 Important Deleted for CBSE Board 2025 Exams

Theorem 6.9 Deleted for CBSE Board 2025 Exams

Last updated at April 16, 2024 by Teachoo

Theorem 6.4 (SSS Criteria) : If in two triangles, sides of one triangle are proportional to (i.e., the same ratio of) the sides of the other triangle, then their corresponding angles are equal and hence the two triangle are similar. Given: Triangle βABC and βDEF such that π΄π΅/π·πΈ = π΅πΆ/πΈπΉ = πΆπ΄/πΉπ· To Prove: β A = β D, β B = β E, β C = β F and βABC ~ βDEF Construction: Draw P and Q on DE & DF such that DP = AB and DQ = AC respectively and join PQ. Proof: Given π΄π΅/π·πΈ = πΆπ΄/π·πΉ And DP = AB, DQ = AC π·π/π·πΈ = π·π/π·πΉ π·πΈ/π·π = π·πΉ/π·π Subtracting 1 on both sides π·πΈ/π·π β 1 = π·πΉ/π·π β 1 (π·πΈ β π·π)/π·π = (π·πΉ β π·π)/π·π ππΈ/π·π = ππΉ/π·π π·π/ππΈ = π·π/ππΉ Using Theorem 6.2 : If a line divides any two sides of a triangle in the same ratio, then the line is parallel to third side. β΄ PQ β₯ EF. Now, For lines PQ & EF, with transversal PE β P = β E For lines PQ & EF, with transversal QF β Q = β F In βDPQ and βDEF β P = β E β Q = β F β βDPQ ~ βDEF β΄ π·π/π·πΈ = π·π/π·πΉ = ππ/πΈπΉ Also, π΄π΅/π·πΈ = π΅πΆ/πΈπΉ = πΆπ΄/πΉπ· & AB = DP & CA = QD Thus, π·π/π·πΈ = π΅πΆ/πΈπΉ = π·π/πΉπ· From (2) and (3) π΅πΆ/πΈπΉ = ππ/πΈπΉ β΄ BC = PQ In βABC and βDPQ AB = DP AC = DQ BC = PQ β βABC β βDPQ β΄ β B = β P β C = β Q β A = β D But From (1) β P = β E and β Q = β F Therefore, β B = β P = β E and β C = β Q = β F Therefore, In Ξ ABC & Ξ DEF β B = β E β C = β F β΄ βABC ~ βDEF Hence Proved