Hello all, Here is my problem. I have a very long product of cosines with arguments of the form k*Pi, k being integers. I want to convert this expression to a sum of cosines. TrigReduce works fine, except that it converts cos to sin. So, I want all the sin (after the TrigReduce) to be converted to cos and the arguments be of the form p*Pi, p being rational of course. It has something to do with Hold or HoldAll, but I don't know the details. Or perhaps the TrigReduce can be overridden somehow. Can someone help? Thank's for your time.