4-1.Complex numbers
medium

If $z$ is a complex number such that $\frac{{z - 1}}{{z + 1}}$ is purely imaginary, then

A

$|z|\, = 0$

B

$|z|\, = 1$

C

$|z|\, > 1$

D

$|z|\, < 1$

Solution

(b) Let $\frac{{z – 1}}{{z + 1}} = iy$ where $y \in R$
This gives $z = \frac{{1 + iy}}{{1 – iy}} = \frac{{1 + iy}}{{1 – iy}} \times \frac{{1 + iy}}{{1 + iy}} = \frac{{(1 – {y^2}) + 2iy}}{{1 + {y^2}}}$
$\therefore $ $|z| = \frac{1}{{1 + {y^2}}}\sqrt {{{(1 – {y^2})}^2} + 4{y^2}} = \frac{{1 + {y^2}}}{{1 + {y^2}}} = 1$.

Standard 11
Mathematics

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