A complex number z is given in the form:
z = (x.y) = (realpart) x + imaginary part (iy)
In this case:
z1 = -7i
z2 = 3+3i
To write in trigonometric form:
[tex]\begin{gathered} z\text{ = r\lparen cos}\theta\text{ + isin}\theta) \\ For\text{ z1} \\ r\text{ = }\sqrt{0^2+7^2} \\ \text{ = 7} \\ \theta\text{ =}\tan^{-1}(\frac{7}{0} \\ Since\text{ t}he\text{ }argument\text{ }is\text{ }undefined\text{ }and\text{ y is negative,} \\ \theta=\text{ }\frac{3\pi}{2} \\ In\text{ trig form:} \\ z1\text{ = 7\lparen cos}\frac{3\pi}{2};sin\frac{3\pi}{2}) \\ For\text{ z2} \\ r\text{ = }\sqrt{3^2\text{ +3}^2} \\ \text{ =3}\sqrt{2} \\ \theta\text{ = }\tan^{-1}\frac{3}{3} \\ =\text{ }\frac{\pi}{4} \\ In\text{ trig form:} \\ z2\text{ = 3}\sqrt{2}(cos\frac{\pi}{4};sin\frac{\pi}{4}) \end{gathered}[/tex]
Multiplication in trigonometric form:
[tex]z1*z2\text{ = \lparen21}\sqrt{2}\text{ \rparen \lparen cos}\frac{7\pi}{4};\text{ sin}\frac{7\pi}{4})[/tex]
Multiplication in standard form:
[tex]\begin{gathered} (-7i)(3\text{ + 3i\rparen} \\ =-21i\text{ - 21i}^2 \\ i^2\text{ = -1} \\ =\text{ -21i + 21} \\ r\text{ = }\sqrt{21^2+21^2} \\ =21\sqrt{2} \end{gathered}[/tex]
