z=a+bi=r(cosθ+isinθ)
r=√a2+b2, θ=tan−1(ba)
r1(cos(θ1)+isin(θ2))+r2(cos(θ2)+isin(θ2))=r1cos(θ1)+r2cos(θ2)+i(r1sin(θ1)+r2sin(θ2))
r1=√−92+12)=√82
r2=√−12+32=√10
θ1=tan−1(1−9)≈173.66∘, II quadrant
θ2=tan−1(3−1)≈108.43∘, II quadrant
z1+z2=√82cos(173.66)+√10cos(108.43)+i(√82sin173.66+√10sin108.43)
⇒−9−1+i(1+3)
⇒−10+4i