fft - Using fourier coefficients to reconstruct data in matlab

When doing a discrete fourier transform on some data using matlab's fft function, its output is a set of fourier coefficients but I was wondering how do I go about converting these into an and bn so I can reconstruct the signal using sines and cosines.

E.g. I'd have a for loop that continually adds up a_i cos(ix) + b_isin(ix), where i = 1:N