1. The problem statement, all variables and given/known data
I was looking up complex numbers and the guy on YouTube made something similar to this equation:
i=-1
c^2-(d^2*i^2) = c^2+d^2

( - 2:55)

2. Relevant equations
I do not understand why it is "c^2+d^2" and not "c^2-d^2"
I would like a detailed explanation, as I might have misunderstood algebra somehow

3. The attempt at a solution
I would do this to find a solution
c^2-(d^2*i^2)
c^2-(d^2*(-1)^2)
c^2-(d^2*1)
c^2-d^2