Let ABC a right triangle with ∠ A= 90° and let AH = h, the height of Δ ABC.
Area of Δ ABC = (base x height)/2 .
Area of Δ ABC = (BC x h)/2 . But h = 2.BC, then plugin the value of h
Area of Δ ABC = (BC x 2.BC)/2 .
Area of Δ ABC = (2.BC²)/2 .
Area of Δ ABC = (BC)²