（x+5y-9）(x-5y+9) =[x+(5y-9)][x-(5y-9)]=x^2-(5y-9)^2=x^2-25y^2+90y-81 (-2a-b)^2(2a-b) = (-2a-b)[(-2a-b)(2a-b)]=(-2a-b)(b^2-4a^2)=-b^3+4a^2b-2ab^2+8a^3 20又4分之3×19又4分之1=83/4*77/4=399又16分之7200...