已知 a"+ b" =50,(a+b)"=8,则
a"+b"+2ab =8
2ab +50 =8
2ab = -42
于是
(a-b)"= a"+b"-2ab
= 50-(-42)= 50+42 =92 =4*23
或者
(a-b)"= a"+b"-2ab = a"+2ab+b"-4ab =(a+b)"-2*(2ab)
= 8-2*(-42)= 8+84 =92 =4*23
(a+b)"=8,可是(a-b)"=92,我们就要想想,a+b 和a-b,到底是正数还是负数呢?
a-b = 2根号23,或者 a-b = -2根号23,你自己决定吧.