http://img46.imageshack.us/img46/396/imag0035q.jpg

So... I've got the above linear equation system. I need to find alpha1 and alpha2, the rest is assumed as known. I was thinking of solving it by solving first equation for sina1 which equals to (x1-x2-d2*cosa2) / d1 and then substitute it into the second equation, but in order to do that I have to change it to cosa1. Looked through all the trigonometric identities, just can't find the solution... Any thoughts?

Thanks!!!