(2+3)-2(x-y)去括号是:2y-2ⅹ+5
(x+y)^3=x^3+3x^2 y+3xy^2+y^3这是和的立方公式2（x+y）³=2x^3+6x^2 y+6xy^2+2y^3
(2+3)-2(x-y)=5-2x+2y
(2+3)-2(x-y）=5-2x+y
