# Doubt from infinite series

**Dishi_2020**#1

**Ankur_2020**#8

Answer is not looking good so i have posted initial steps you can differentiate it and find the result

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**r1sha6h**#9

You have made a slight mistake. The Coeff of x^6 will be 4 not 3. I see what you were thinking. But x^6 will also be formed from x, x^2, x^3. You have only considered coeef which formed from multiplication of 2 terms. But multiplication of 3/4/5.. terms may also form the coeff.