-a^2+2ab-b^2+4=0
a^2-2ab+b^2-4=0
a^2-2ab+b^2=4
(a-b)^2=4
a-b=2或a-b=-2
长方形的周长是16m
2*（a+b）=16
a+b=8
___________________
a+b=8
a-b=2
___________a=5,b=3_____________
a+b=8
a-b=-2
__________a=3,b=5______________
a*b都是3*5=15 (m^2)