Question: prove triangels are similar if their side are respectivel parallel to eac other? 

Answer: By the definition of Similar triangles - "Two Triangles are similar if their respective angles are the equal".\nHence if you have two triangles (A and B) whose respective sides are parallel, then, it means that either triangles A and B are identical or that A is a magnified or diminished version of B. In either case, their angles will be equal hence making them "Similar"\nTo test this. Draw a triangle (A) on a sheet of paper. Try to draw lines parallel to the sides of triangle A. Ensure that the angles of the new triangle (B) that you are drawing are the exact same as A. You will notice that B is either the exact same as A, A zoomed out or zoomed in version of triangle A
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