Expansion of a periodic function of period L

If f(x) is a function of period L, then its Fourier series is given by:(1) f ( x ) = a 0 + ∑ n = 1 ∞ [ a n c o s ( 2 π n x L ) + b n s i n ( 2 π n x L ) ] https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429294402/968186b9-ffa4-4f2b-aa54-7fc58a604561/content/eq6020.tif"/>