You need to test if the first derivative of the function is positive, hence, you need to differentiate the given function with respect to `x` , such that:
`f_n'(x) = (int_0^x t^n*sqrt(t^2 + 1)dt)' => f_n'(x) = x^n*sqrt(x^2 + 1)`
Since `x^2 > 0, AA x in R => x^2 + 1 > 0 => sqrt(x^2 + 1)>0` .
You need to notice though that the problem does not provide information about the domain of definition of the given function, because if the domain would be `[0,oo)` , then the derivative is strictly positive, but if the domain would be the set `R` , then the derivative is negative over `(-oo,0).`
Hence, testing if the given function increases, yields that the statement is valid only if `x >=0` .