And here’s how:
-1 = -1
-1/1 = -1/1
-1/1 = 1/-1
Take the square root of both sides.
sqrt(-1/1) = sqrt(1/-1)
i/1 = 1/i
i = 1/i
i^2 = 1
-1 = 1
Ok, I know. I just did 2=1. This one is a little harder to spot, though.
Edit: LahDsai found the error, congrats. I should have 1/i^2 after multiplying by i on both sides.
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I can prove that 1+1=4. So, you add 1 and 1 to get 2. Then you break the plus sign into two ones and add them to the two. You get 4.