令A的横坐标为a, 3a + y - 2 = 0, 其纵坐标为2 - 3a令B的横坐标为b, b - 3y + 2= 0, 其纵坐标为(b + 2)/3P(1, 5)为AB的中点, 则(a + b)/2 = 1[(2 - 3a) + (b + 2)/3]/2 = 5二者联立，得a = -2, b = 4A(-2, 8), B(4, 2)tanα = (8 - 2)/(-2 - 4) = -1α = 135°
