What have you tried? Do you know the power series for ? Have you tried just plugging that in and simplifying?
Yes i did, i got series (-1)^(n+1) *x^(2n-2) (series from n=1 to infinity)
But it was not the correct answer
The correctly answer is ∑k=0∞((−1)^k)/(2k+3)*x^(2k)
I do not understand how he get this answer. Have you any suggestion?