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)(α−β) 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+βα−β 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+cosB−i(sinA+sinB) =cos(A−B2)[cos(A+B2)+isin(A+B2)]−sin(A−B2)[sin(A+B2)−icos(A+B2)] Z=−icos(A−B2)sin(A−B2) Therefore, Z is purely imaginary.