Supposing the area of DeltaABC is given by (b_1 h_1)/2 and the area of DeltaPQR is given by (b_2h_2)/2 If they are similar by lambda = a/b the their areas are similar by lambda^2 because
(a_2b_2)/2=((lambda a_1)(lambda b_1))/2=lambda^2(a_1,b_1)/2 so finally