If the sum of the first 10 terms of the series $\frac{4 \cdot 1}{1+4 \cdot 1^4}+\frac{4 \cdot 2}{1+4 \cdot 2^4}+\frac{4 \cdot 3}{1+4 \cdot 3^4}+\ldots .$. is $\frac{\mathrm{m}}{\mathrm{n}}$, where $\operatorname{gcd}(\mathrm{m}, \mathrm{n})=1$, then $\mathrm{m}+\mathrm{n}$ is equal to _______________
Answer (integer)
441
Solution
<p>$$\begin{aligned}
& \frac{4.1}{1+4.1^4}+\frac{4.2}{1+4.2^4}+\frac{4.3}{1+4.3^4}+\ldots . \\
& T_r=\frac{4 r}{1+4 r^4}=\frac{4 r}{4 r^4+4 r^2+1-4 r^2} \\
& =\frac{4 r}{\left(2 r^2+1\right)^2-(2 r)^2} \\
& T_r=\frac{4 r}{\left(2 r^2-2 r+1\right)\left(2 r^2+2 r+1\right)}
\end{aligned}$$</p>
<p>$$\begin{aligned}
& T_r=\frac{\left(2 r^2+2 r+1\right)-\left(2 r^2-2 r+1\right)}{\left(2 r^2-2 r+1\right)\left(2 r^2+2 r+1\right)} \\
& T_r=\left(\frac{1}{r^2+(r-1)^2}-\frac{1}{r^2+(r+1)^2}\right) \\
& \sum_{r=1}^{10} T_r=\left(\frac{1}{0^2+1^2}-\frac{1}{1^2+2^2}+\frac{1}{1^2+2^2}-\frac{1}{2^2+3^2}+\ldots .\right. \\
& \frac{1}{9^2+10^2}-\frac{1}{10^2+11^2} \\
& =1-\frac{1}{221} \\
& =\frac{220}{221} \\
& \therefore \quad m+n=220+221 \\
& \quad=441
\end{aligned}$$</p>
About this question
Subject: Mathematics · Chapter: Sequences and Series · Topic: Arithmetic Progression
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