解：a(a+b)(a-b)-a(a+b)的平方 =a(a^2-b^2)-a(a^2+2ab+b^2) =a^3-ab^2-a^3-2a^2b-ab^2 =-2a^2b-2ab^2 =-2ab(a+b) =-2*(-5/2)*(-2) =-10
