Question: (1)$1=\sqrt{1}$

(2)$=\sqrt{-1×-1}$

(3)$=\sqrt{-1}×\sqrt{-1}$

(4)$=i×i=-1$

$1=-1$

Which step is wrong? I guess step 3 but I dont know why

Answer: 
The third step because $$\sqrt{-1}=\{i,-i\}$$ and we can not write $$\sqrt{(-1)(-1)}=i\cdot i.$$
In $\mathbb C$, $$\sqrt{ab}=\sqrt a\sqrt b,$$
doesn't hold.