i = √−1 · i² = −1
z = x + yi
Add: real+real, imag+imag
Multiply like brackets, i² → −1
Conjugate of x+yi is x−yi
Divide: times top & bottom by conjugate
Argand: z = x+yi → point (x, y)
|z| = √(x² + y²)
Equal: real=real, imag=imag
Complex roots come in pairs: x±yi
Prove a root: sub in, get 0