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Basic Proportionality Theorem ( Thales Theorem ) and its converse
Theorem: If a line is drawn parallel to one side of a triangle to intersectthe other two sides in distinct points, the other two sides are divided in the same ratio.
Given: A triangle ABC and DE || BC
Construction: Join BE and CD and draw DM AC and EN
AB
Proof:
Using Eq 1, 2, 3 and 4
Now So ar(BDE) = ar( DEC) Using the relation in Eq 5 and 6 we get Hence Proved | ![]() |
Converse of the basic proportionality theorem is also true,
Theorem If a line divides any twosides of a triangle in the same ratio, the line is parallel to the third side. Given : a triangle ABC and That is, if in fig. To Prove : Construction: Draw DF || BC Proof: If DF || BC then But we are given Then from 1 and 2 we get This is possible only when F and E coincide. This implies DE || BC | ![]() |
Example :.
Solution
As | ![]() |



