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Answers
Basic Proportionality Theorem was first stated by Thales, a Greek mathematician. Hence it is also known as Thales Theorem. Thales first initiated and formulated the Theoretical Study of Geometry to make astronomy a more exact science. What is this theorem that Thales found important for his study of astronomy? Let us find it out.
Basic Proportionality Theorem (can be abbreviated as BPT) states that, if a line is parallel to a side of a triangle which intersects the other sides into two distinct points, then the line divides those sides in proportion.
In the figure alongside, if we consider DE is parallel to BC, then according to the theorem,
ADBD=AECE
PROOF OF BPT
Given: In ΔABC, DE is parallel to BC
Line DE intersects sides AB and PQ in points D and E, such that we get triangles A-D-E and A-E-C.
To Prove: ADBD=AECE
Construction: Join segments DC and BE
Proof:
In ΔADE and ΔBDE,
A(ΔADE)A(ΔBDE)=ADBD (triangles with equal heights)
In ΔADE and ΔCDE,
A(ΔADE)A(ΔCDE)=AECE (triangles with equal heights)
Since ΔBDE and ΔCDE have a common base DE and have the same height we can say that,
A(ΔBDE)=A(ΔCDE)
Therefore,
A(ΔADE)A(ΔBDE)=A(ΔADE)A(ΔCDE)
Therefore,
ADBD=AECE
Hence Proved.
The BPT also has a converse which states, if a line divides any two sides of a triangle in the same ratio, then the line is parallel to the third side.
(Note: A converse of any theorem is just a reverse of the original theorem, just like we have active and passive voices in English.)
