因式分解(点等于平方)
ax`+bx+c=a{x`-(x1+x2)x+x1x2}
=a(x-x1)(x-x2)
是怎么演变的?x1和x2是x的两个解.
a[x^2-(x1+x2)x+x1x1]怎么演变到后面的?=a(x-x1)(x-x2)