Accedmychevron_right11thchevron_rightmathchevron_rightComplex Numberschevron_rightExercise 1.2

(i)

Solve:

x+iy+23i=i(5i)(3+4i)x+iy+2-3i=i(5-i)(3+4i)

Solution

First multiply (5i)(3+4i):(5i)(3+4i)=15+20i3i4i2=15+17i+4=19+17iNow multiply by i:i(19+17i)=19i+17i2=17+19iEquate real and imaginary parts:x+iy+23i=17+19i(x+2)+i(y3)=17+19ix+2=17x=19y3=19y=22\begin{aligned} & \boxed{\text{First multiply }(5-i)(3+4i):}\\ \\ & (5-i)(3+4i)=15+20i-3i-4i^2 \\ &=15+17i+4=19+17i \\ \\ & \boxed{\text{Now multiply by }i:}\\ \\ & i(19+17i)=19i+17i^2=-17+19i \\ \\ & \boxed{\text{Equate real and imaginary parts:}}\\ \\ & x+iy+2-3i=-17+19i \\ & (x+2)+i(y-3)=-17+19i \\ \\ & x+2=-17 \Rightarrow x=-19 \\ & y-3=19 \Rightarrow y=22 \end{aligned} x=19, y=22\boxed{x=-19,\ y=22}