JEE MAIN - Mathematics (2025 - 4th April Morning Shift - No. 7)
$1+3+5^2+7+9^2+\ldots$ upto 40 terms is equal to
40870
41880
43890
33980
Explanation
$$\begin{aligned}
& 1+3+5^2+7+9^2+\ldots \text { upto } 40 \text { terms } \\
& \left(1^2+5^2+9^2+\ldots\right)+(3+7+11+\ldots) \\
& =\left(\sum_{k=1}^{20}(4 k-3)^2\right)+\frac{20}{2}[6+(20-1) 4] \\
& =16 \sum_{k=1}^{20} k^2-24 \sum_{k=1}^{20} k+9 \times 20+10[82] \\
& =16\left(\frac{20 \times 21 \times 41}{6}\right)-24\left(\frac{20 \times 21}{2}\right)+1000 \\
& =45920-5040+1000 \\
& =41880
\end{aligned}$$
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