乘法公式chengfa gongshi
包括：
❶ (x+y)2＝x2+2xy+y2.
❷ (x-y)2＝x2-2xy+y2.
❸ (x+y)(x-y)=x2-y2.
❹(x+y)3＝x3+3x2y+3xy2+y3.
❺(x-y)3＝x3-3x2y+3xy2-y3.
❻(x-y)(x2+xy+y2)=x3-y3.
❼(x+y)(x2-xy+y2)=x3+y3.
❽(x+a)(x+b)=x2+(a+b)x+ab.
❾(x1+x2+…+xn)2＝x21+x22+…+x2n+2x1x2+2x1x3+…+2xn-1xn.
❿ (x-y)(xn-1+xn-2y+…+xyn-2+yn
＝xn-yn.
(11)(x+y)(x2k-1-x2k-2y+x2k-3y2-…-y2k
)=x2k-y2k.
(12)(x+y)(x2k-x2k-1y+x2k-2y2-…+y2k)=x2k+1+y2k+1.
乘法公式chengfa gongshi
参见 “条件概率”。