Construct triangle BCD, and then construct a perpendicular from D meeting BC at point E, let DE be the height of the triangle and BC be the base of that triangle. Therefore we can state that the area of the triangle is 0.5(BC)*(DE). Construct

segment DE and find the mid-point F as shown above. Construct a rectangle with length GH = BC and HJ = EF . Finally use the rectangle to construct square of same area as that of triangle. It will be noted that that the area of the triangle, rectangle and square are equal. click here for the proof.