Show that the triangle with vertices P(1,2,2), Q(7,-1,-1) and R(3,0,8) is right-angled.My notes tell me that a triangle is only right angled if p.q = 0, but p.q = -11, p.r = 19, q.r = -29. I also tried to find out the individual angles and I got PQ = 120, PR = 42, QR= 118.

Am I making an obvious mistake or just doing something stupidly wrong? Thanks for your help :]