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Exercise 432Algebra › Complex numbersHigh schoolAdded 3 Nov 2026

Product of complex numbers in trigonometric form

#trigonometric form#product#quotient

Statement

Let z1=2(cos⁡π6+isin⁡π6)z_1=2(\cos\frac\pi6+i\sin\frac\pi6) and z2=3(cos⁡π3+isin⁡π3)z_2=3(\cos\frac\pi3+i\sin\frac\pi3). Compute z1z2z_1z_2 and z1/z2z_1/z_2 in trigonometric form.

Hint

Moduli multiply/divide; arguments add/subtract.

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Solution

Product. ∣z1z2∣=2⋅3=6|z_1z_2|=2\cdot3=6, arg⁡(z1z2)=π/6+π/3=π/2\arg(z_1z_2)=\pi/6+\pi/3=\pi/2. So
z1z2=6(cos⁡π2+isin⁡π2)=6i.z_1z_2=6(\cos\frac\pi2+i\sin\frac\pi2)=6i.

Quotient. ∣z1/z2∣=2/3|z_1/z_2|=2/3, arg⁡(z1/z2)=π/6−π/3=−π/6\arg(z_1/z_2)=\pi/6-\pi/3=-\pi/6. So
z1z2=23(cos⁡(−π6)+isin⁡(−π6))=23(32−i2)=33−i3.\frac{z_1}{z_2}=\frac23\left(\cos\left(-\frac\pi6\right)+i\sin\left(-\frac\pi6\right)\right)=\frac23\left(\frac{\sqrt3}{2}-\frac i2\right)=\frac{\sqrt3}{3}-\frac i3.

Conclusion. z1z2=6iz_1z_2=6i and z1/z2=33−i3z_1/z_2=\frac{\sqrt3}{3}-\frac{i}{3}.