18. /find the radius of convergence of 2 (2n)! 1 (1- 19 find the sum of the coefficients of x20 and x21 in the power series expansion of 2 (ava

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Ans is 120 as x^20 coefficient is zero so now we just have to calculate coefficient of x^21 = (x^3)^7, so numerator has 7 terms starting from 4 and in denominator we have 7! So (4...

Refer attachment , answer is 120

Answer is 120 . See complete solution below