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4-1.Complex numbers
medium
योगफल $\sum\limits_{n = 1}^{13} {({i^n} + {i^{n + 1}})} $, जहाँ $i = \sqrt { - 1} $ है, का मान है
A
$i$
B
$i - 1$
C
$ - i$
D
$0$
(IIT-1998)
Solution
(b) $\sum\limits_{n = 1}^{13} {({i^n} + {i^{n + 1}})} $$ = (i + {i^2} + {i^3} + …. + {i^{13}}) + ({i^2} + {i^3} + …. + {i^{14}})$
$ = \frac{{i(1 – {i^{13}})}}{{1 – i}} + \frac{{{i^2}(1 – {i^{13}})}}{{1 – i}} = i\,\left( {\frac{{1 – i}}{{1 – i}}} \right) + \frac{{{i^2}(1 – i)}}{{(1 – i)}}$
$ = i + {i^2} = i – 1$.
Standard 11
Mathematics