Hi,
This is allegedly a very simple question....most of the people answered that the answer is 9=90. i claim that theoretically, you can't be 100% sure about the right answer because you don't know how the function\series behaves at higher numbers, but most other people claim that this is a definite series in which you can know the matching numbers up to infinity.
Hope you are able to understand exactly what i meant. So.....who's right?
Whoever made the problem does not understand the $=$ sign the same way we do. What they really mean is $f(2)=6$ and so on. You are absolutely correct that you can't be sure what the function does for other arguments. You are expected to find the "simplest" answer (as defined by the setter). I agree with most that for this problem $90$ is the intended answer.