Question

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Answer 1

should be (-inf,inf) since cos(x) is convergent at every x, and cos(x^2) is basically the same formula with x replaced with x^2

Answer 2

Are you sure?

Answer 3

you can use limit ratio test to verify

Answer 4

or wolfram alpha

Answer 5

I was trying to do that but it like, said it diverges??

Answer 6

https://www.wolframalpha.com/input/?i=sum+%7B%5B%28%28-1%29%5E%28n%29%29*%28%28x%C2%B2%29%5E%282n%29%29%5D%2F%28%282n%29%21%29%7D+from+0+to+inf

Answer 7

oh wow, I must have been looking in the wrong place then. Thanks!