a²+b²-4a+2b+5
=(a²-4a+4)+(b²+2b+1)
=(a-2)²+(b+1)²=0
(a-2)²>=0,(b+1)²>=0
两个非负数和为0,则每个都等于0
a-2=0
b+1=0
a=2,b=-1
(1+ab)²=(1-2)²=1
(1+ab)²的平方根=±1