If a line divides any two sides of a triangles proportionally then the line is parallel to the third side Statement: If a line divides any two sides of a triangles proportionally (in same ratio), then the line is parallel to the third side. Proof: We are given ABC AD / BD = AE / CE We have to prove: DE is parallel to BC Let DE's not parallel to BC then an another line DE' is parallel to BC. Now AD / BD = AE / CE [Given] And AD / BD = AE' / CE' [ Thales Theorem ] Therefore AE / CE = AE' / CE' i.e. AE / CE + 1 = AE' / CE' + 1 i.e. AE + CE / CE = AE' + CE' / CE' i.e. CE = CE' But this is not possible until E and E' is coincident. Thus, our assumption is wrong and DE is parallel to BC. X Triangles :Criteria for Similarity of Triangles Solved: 10th math Triangles Thales theorem 10th math Converse of Thales theorem Angle bisector theorem 10th math SAS similarity of triangle 10th SSS similarit...
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