Sum of products of corresponding terms of sequences 2,4,8,16,32 and 128,32,8,2, frac{1}{2}

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8. Sequences and Series

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Find the sum of the products of the corresponding terms of the sequences $2,4,8,16,32$ and $128,32,8,2, \frac{1}{2}$

$496$

Solution

Required sum $=2 \times 128+4 \times 32+8 \times 8+16 \times 2+32 \times \frac{1}{2}$

$=64\left[4+2+1+\frac{1}{2}+\frac{1}{2^{2}}\right]$

Here, $4,2,1, \frac{1}{2}, \frac{1}{2^{2}}$ is a $G.P.$

First term, $a=4$

Common ratio, $r=\frac{1}{2}$

It is known that, $S_{n}=\frac{a\left(1-r^{n}\right)}{1-r}$

$\therefore S_{5}=\frac{4\left[1-\left(\frac{1}{2}\right)^{5}\right]}{1-\frac{1}{2}}=\frac{4\left[1-\frac{1}{32}\right]}{\frac{1}{2}}=8\left(\frac{32-1}{32}\right)=\frac{31}{4}$

$\therefore$ Required sum $=64\left(\frac{31}{4}\right)=(16)(31)=496$

Standard 11

Mathematics

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Solution

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Find the sum of first $n$ terms and the sum of first $5$ terms of the geometric
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