Concept understanding — Modulus and Argument of a Complex Number
For a complex number z=a+ib represented by the point P(a,b) in the Argand plane, the modulus ∣z∣=r=a2+b2 is the straight-line distance OP from the origin to P (found via Pythagoras on the right triangle formed by the point and the two axes), and the argument θ=arg(z) is the angle that OP makes with the positive real axis, satisfying cosθ=a/r, sinθ=b/r, and tanθ=b/a when a=0. Because the inverse tangent function alone only ever returns an angle in (−π/2,π/2), it cannot on its own distinguish a point in quadrant I from one in quadrant III (or quadrant II from quadrant IV), so finding the argument in the standard range 0≤θ<2π requires checking which quadrant (or axis) the point (a,b) actually lies in and adding the appropriate correction: no correction in quadrant I, +π in quadrants II and III, and +2π in quadrant IV. The modulus obeys clean multiplicative rules — ∣z1z2∣=∣z1∣∣z2∣, z2z1=∣z2∣∣z1∣, and zzˉ=∣z∣2 — together with the triangle inequality ∣z1+z2∣≤∣z1∣+∣z2∣, while the argument obeys additive rules — arg(z1z2)=argz1+argz2 and arg(z1/z2)=argz1−argz2 — which together are the algebraic seeds of De Moivre's theorem.
For z=3+2i: ∣z∣=3+2=5. Both a,b>0, Quadrant I, so argz=tan−1(32)=tan−132, which lies between 0 and π/2 as expected.\n> [!ANSWER] ∣z∣=5, argz=tan−12/3.
Quadrant I needs no correction: argz=tan−1(b/a) directly.