1. The problem statement, all variables and given/known data The vector a=2 and vector b=1. The vectors a+5b and 2a-3b are perpendicular. Determine the angle between a and b . 2. Relevant equations The dot product a•b=lallblcosθ 3. The attempt at a solution I've tried a few things but none of it really makes sense. I'm worried that maybe this question doesn't call for the dot product method but I've become fixed on it.