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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