Eric Nicholas K's answer to Olivia Alexander's Secondary 3 A Maths Indonesia question.

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Eric Nicholas K
Eric Nicholas K's answer
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There is a shorter method than this though.

I went to the trouble to prove that AOMN is a parallelogram so that I can use the two fact that AO is parallel to MN and AN is parallel to OM to prove that all the triangles are congruent.

Coming next post is the final part to the explanation.