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प्रश्न
The value of \[\sum^n_{r = 1} \left\{ (2r - 1) a + \frac{1}{b^r} \right\}\] is equal to
विकल्प
\[a n^2 + \frac{b^{n - 1} - 1}{b^{n - 1} (b - 1)}\]
\[a n^2 + \frac{b^n - 1}{b^n (b - 1)}\]
\[a n^3 + \frac{b^{n - 1} - 1}{b^n (b - 1)}\]
none of these
उत्तर
\[a n^2 + \frac{b^n - 1}{b^n (b - 1)}\]
We have:
\[\sum^n_{r = 1} \left\{ (2r - 1) a + \frac{1}{b^r} \right\}\]
\[ = \sum^n_{r = 1} \left\{ 2ra - a + \frac{1}{b^r} \right\}\]
\[ = \sum^n_{r = 1} 2ar - \sum^n_{r = 1} a + \sum^n_{r = 1} \frac{1}{b^r}\]
\[ = an\left( n + 1 \right) - an + \frac{\left( 1 - b^n \right)}{\left( 1 - b \right) b^n}\]
\[ = a n^2 + \frac{\left( b^n - 1 \right)}{\left( b - 1 \right) b^n}\]
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