1. The problem statement, all variables and given/known data In the parallelogram ABCD the internal bisectors of the consecutive angles B and C intersect at P. Use vector method to find angle BPC. 2. Relevant equations 3. The attempt at a solution I assume angle BPC to be θ and the point of intersection of internal bisectors to be P. But what is the next operation which I should use? I tried taking dot product of PB and PC but I don't yet know the magnitudes of both vectors.