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Question

Z1Z2 are two points in an argand plane. If a|Z1|=b|Z1|
Prove that aZ1bZ2aZ1+bZ2 is purely imaginary.

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Solution

Let Z1=r1eiθ,Z2=r2ei(θ+α).
Given that ar1=br2
Z=aZ1bZ2aZ1+bZ2=eiθei(θ+α)eiθ+ei(θ+α)
=1eiα1+eiα (Dividing Nr. and Dr. by eiθ)
=eiα/2eiα/2eiα/2+eiα/2 (Dividing Nr. and Dr. by eiα/2)
=2isinα22 cosα2
=itanα2
Hence,Z is purely imaginary
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