1. The problem statement, all variables and given/known data The question si the make a rotation of axis to elmininate the cross-product term in the work i provided in this pic: http://img295.imageshack.us/img295/9579/untitledtu4.png [Broken] 2. Relevant equations 3. The attempt at a solution I've drawn of the example in the pic i've given a link. my questions: how do i know that 2 theta = pi/3 and theta is pi/6 where does the sqrt(3)/2 and 1/2 come from? i know there is some kind of 30/60/90 triangle going on here i think. can someone please help me understand where the numbers are coming from. if you expand the pic its MUCH easier to view. thankyou.