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Question

Let α and β be two distinct complex numbers such that |α|=|β|. If real part of α is positive and imaginary part of β is negative, then the complex number (α+β)(αβ) may be

A
zero
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B
real and negative
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C
real and positive
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D
purely imaginary
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Solution

The correct option is C purely imaginary
Let Z= α+βαβ
cis(θ)=cosθ+isinθ
Let, |α|=|β|=r&α=rcis(A),β=rcis(B)
Z=cis(A)+cis(B)cis(A)cis(B)
= cosA+cosB+i(sinA+sinB)cosA+cosBi(sinA+sinB)
= cos(AB2)[cos(A+B2)+isin(A+B2)]sin(AB2)[sin(A+B2)icos(A+B2)]
Z=icos(AB2)sin(AB2)
Therefore, Z is purely imaginary.

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