Hi Folks, Got a question regarding how to calculate the Fourier coefficents a0, an and bn. I have a problem with the orthogonal trig identities which are used to identify the particular co-efficent. I know it works and I can just take that and accept, however I would like to see a proof of it. The identitiy is as follows: I dont now how to show the integral sign so I will use (int instead) sorry! 2*pi int cos mx. cos nx dx = 0 ( if m does not equal n) 0 = 1 ( if m does equal n(therefore identifying term)) Identity used cos mx. cos nx = = 1/2 (cos(m+n)x + cos(m-n)x) substuting this identity into the integral above. 2*pi 1/2 int [(cos(m+n)x + cos(m-n)x)]dx 0 = | sin(m+n)x sin(m-n)x | 2*pi | ---------- + ---------- | | (m+n) (m-n) |0 I finally get to my question According to the orthogonal feature of this, the result will be 1 only when m=n all other values of m and n will result in the reslut bieng 0. The thing I cant understand is taht in each part of the integral the sin term is been multiplied by a multiple of 2*pi or 0, surly any multiple of that result in a zero output. So how is it possible to get a result of 1 when m equals n. I know I am wrong but I would appreciate it if someone will tell me where I am going wrong. I have tried doing it on my calculator and it always works out to be 0. Jag.