有一个公式是(x+y)²=x²+2xy+y²
那么(x+y)³=?
(x+y)^3
=(x+y)^2(x+y)
=(x²+2xy+y²)(x+y)
=x^3+x^2y+2x^2y+2xy^2+xy^2+y^3
(x+y)³=(x-y)(x²+xy+y²) (x-y)³=(x²+y²)(x²-xy+y²)
x³+3x²y+3xy²+y³
系数符合杨辉三角形