The Joiner
Answer :

Nothing could be easier than the solution of this puzzle—when you know how to do it. 
And yet it is apt to perplex the novice a good deal if he wants to do it in the fewest possible pieces—three. 
All you have to do is to find the point A, midway between B and C, and then cut from A to D and from A to E. 
The three pieces then form a square in the manner shown. 
Of course, the proportions of the original figure must be correct; thus the triangle BEF is just a quarter of the square BCDF. 

Draw lines from B to D and from C to F and this will be clear.

Math Genius