MATLAB: Doubt fft with values

Question

If you see, there is sin( 2 pi 15* t) signal and I want to show the spectrum of it. The problem what I see it's that there are 2 impulses, ones on 15 Hz (it's right) but other on 35 Hz. And the other problem it's that the signal magnitude is approximately 200 but the signal has an amplitud of 1.

Answer 1

Thanks. I work!, but I do not understand why I should divide the fft(x)'s vector by lenght(x).

For example in the next picture, the max lobule must to be on 100 Hz. why not is there?