(x+1)(y-1)
=xy-x+y-1-x+y=-(x-y)
=xy-(x-y)-1
=-√2-5√2+1-1
=-6√2-x+y=-(x-y)怎么变成这样的？？？？？这是添括号放到带有负号的括号里，各项都要变号