Part of ALG-02 — Complex Numbers

Rapid Revision Bullets

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  1. i2i^2 = -1, powers cycle: 1, i, -1, -i (period 4)
  2. z*z-bar = |z|^2 -- THE most important identity
  3. z + z-bar = 2Re(z), z - z-bar = 2i*Im(z)
  4. |z1z2| = |z1||z2|; arg(z1*z2) = arg(z1)+arg(z2)
  5. Euler: e^(itheta) = cos(theta) + isin(theta)
  6. De Moivre: (cis theta)^n = cis(n*theta)
  7. nth roots: n equally spaced points on circle, sum = 0
  8. Cube roots: 1+w+w2w^2 = 0, w3w^3 = 1
  9. 1+w = -w2w^2, 1+w2w^2 = -w (use immediately)
  10. |z-z0| = r is a circle; |z-z1| = |z-z2| is perpendicular bisector
  11. Max|z| = |center| + radius; Min|z| = ||center| - radius|
  12. Triangle inequality: ||z1|-|z2|| <= |z1+z2| <= |z1|+|z2|
  13. arg(z-bar) = -arg(z); arg in (-pi, pi]
  14. Complex roots of real polynomials come in conjugate pairs
  15. sqrt(-a)*sqrt(-b) = -sqrt(ab), NOT sqrt(ab)
  16. On unit circle: 1/z = z-bar
  17. z+1/z = 2cos(theta) when z = e^(i*theta)
  18. Rotation by alpha about origin: multiply by e^(i*alpha)
  19. z = x+iy and separate parts for z + z-bar equations
  20. Equilateral triangle: z1^{2+z2}^{2+z3}^2 = z1z2+z2z3+z3z1

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