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Circles

Question
CBSEENMA9002770

In triangle ABC, points M and N on sides AB and AC respectively are taken so that 

AM equals 1 fourth AB space and space AN equals 1 fourth AC comma space Prove space that space MN equals 1 fourth

Solution
Given: In triangle ABC, points M and N on the sides AB and AC respectively are taken so that
AM space equals 1 fourth AB space and space AN equals 1 fourth A C
To prove: MN equals 1 fourth BC.

Construction: Join EF where E and F are the middle points of AB and AC respectively.
Proof: Y E is the mid-point of AB and F is the mid-point of AC.

therefore space space EF space parallel to space BC space and space space space space space space EF equals 1 half BC space space space space space space space space space space space space space... left parenthesis 1 right parenthesis
space space space Now comma space space space space space space space space space space space space space space space space space space space AE equals 1 half AB
space space space and space space space space space space space space space space space space space space space space space space space space AM equals 1 fourth space AB
therefore space space space space space space space space space space space space space space space space space space space space space space space space space AM space equals space 1 half AE
Similarly comma space space space space space space space space space space space space space space space space AN space equals space 1 half space AF
rightwards double arrow space space M and N are the mid-points of AE and AF respectively.
therefore space space MN space parallel to space EF space and space MN space equals space 1 half space EF equals 1 half open parentheses 1 half BC close parentheses
                                                             | From (1)
                                             equals space 1 fourth space BC.