Use vectors to decide whether the triangle with vertices p (1,-3,-2), Q (2,0,4), R (6,-2,-5) is right-angled.

The direction vectors for the three sides are given by the differences in the vertices, not the vertices themselves. So consider the vectors

P-Q, P-R, Q-RP-Q, P-R, Q-Rand see whether any of these are orthogonal.