請證明如果x^2=y^2，x一定等如y

x^2 -y^2=0
(x+y)(x-y)=0
x= -y or "x=y"

x一定等如y

take square root on both sides, x=y

which mean we hv
x^2=y^2
x=y or x= -y

2010-09-04 18:39:18 補充：
remarks: not only 1 answer

根本就不等於的，舉個反例：
5^2 = (-5)^2
5 = -5 ???