CosB = (AB•BC)/(|AB|•|BC|)
AB = (3-1,3-5,2-3) = (2,-2,-1)
|AB| = sqrt(4+4+1) = sqrt(9) = 3
BC = (3-3,6-3,5-2) = (0,3,3)
|BC| = sqrt(0+9+9) = sqrt(18) = 2sqrt(3)
AB*BC = 2*0 + (-2)*3 + (-1)*3 = -6 - 3 = -9
AB = (3-1,3-5,2-3) = (2,-2,-1)
|AB| = sqrt(4+4+1) = sqrt(9) = 3
BC = (3-3,6-3,5-2) = (0,3,3)
|BC| = sqrt(0+9+9) = sqrt(18) = 2sqrt(3)
AB*BC = 2*0 + (-2)*3 + (-1)*3 = -6 - 3 = -9