Given that the two vectors u=(2a+b) and v=(4a-3b) are perpendicular and that |a|=3 and |b|=6, then find the angle between a and b.
So I pretty sure i need to set the dot product of u and v = to zero. Then there is probably some sort of way to rearange it, of which I am not sure.
thanks for your kind reply BUT: your problem is a little bit more tricky than it seems to be:
1. According to my result the vectors and are pointing in opposite directions, that means they are collinear. And therefore any linear combinations of and must be parallel. Seems to be a contradiction to the given conditions.
2. Considering the directions of and and the length of and then
and everything is nice and peaceful again.