因式分解1.πR^2-πr^2 2.x^3-4x^2+4x 3.-4(x-2y)^2+9(x+y)^2 4.a^3c-4a^2bc+4ab^2c
问题描述：
因式分解1.πR^2-πr^2 2.x^3-4x^2+4x 3.-4(x-2y)^2+9(x+y)^2 4.a^3c-4a^2bc+4ab^2c
①πR^2-πr^2 ②x^3-4x^2+4x ③-4(x-2y)^2+9(x+y)^2 ④a^3c-4a^2bc+4ab^2c
答
πR^2-πr^2=π（R^2-r^2）=π（R+r）(R-r)x^3-4x^2+4x=x(x^2-4x+4)=x(x-2)^2-4(x-2y)^2+9(x+y)^2 =9(x+y)^2 -4(x-2y)^2=(3x+3y)^2-(2x-4y)^2=(5x-y)(x+7y)a^3c-4a^2bc+4ab^2c=ac(a^2-4ab+4b^2)=ac(a-2b)^2