Question

how to construct a triangle equal in area of a rectangle if one angle of triangle is given ​

Answer 1

Take F on the BC-line such that BF=DE. Take G on the AB-line such that CG∥AF and let M be the midpoint of CG. In the following figure, the depicted rectangle and the triangle ABC have the same area:

enter image description here

This happens because the area of the rectangle equals the area of BGF, and since BGBA=BCBF,

[BGF]=[ABC].

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