let s(x) be sqare root of x

-1 = -1
-1/1 = 1/-1
s(-1/1) = s(1/-1)
s(-1)/s(1) = s(1)/s(-1)
multiply with both denominators
s(-1)*s(-1) = s(1)*s(1)
(s(-1))^2 = (s(1))^2
-1 = 1