不用
=a(x1平方-x2平方)+b(x1-x2)
= a(x1+x2)(x1-x2)+b(x1-x2)
=(x1-x2)(a(x1+x2)+b)
=a(x1-x2)((x1+x2)+b/a)
比符号～
当然可以
因为x1 x2都属于(-无穷,-b/2a]
所以x1+x2属于(-无穷,-b/a]
所以((x1+x2)+b/a) 属于(-无穷,0]
其他不用我说了吧
以后遇到问题都思考下吧,这样才好提高嘛