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8. Sequences and Series
easy
गुणोत्तर श्रेणी $5, - \frac{5}{2},\frac{5}{4}, - \frac{5}{8},...$ का $n$ वाँ पद$\frac{5}{{1024}}$ हो, तो $n$ का मान होगा
A
$11$
B
$10$
C
$9$
D
$4$
Solution
(a) $T_n = ar^{n-1}$
$⇒{5 \over {1024}}=5({-1 \over 2})^{n-1}$
==> ${\left( {\frac{{ – 1}}{2}} \right)^{10}} = {\left( {\frac{{ – 1}}{2}} \right)^{n – 1}}$
==> $10 = n – 1$
==> $n = 11$.
Standard 11
Mathematics
