Diketahui P = (a, 0, 3), Q = (0, 6, 5) dan R(2, 7, c). Agar vektor PQ tegak lurus pada QR, maka a - c =

PQ = Q - P
= (0, 6, 5) - (a, 0, 3)
= (-a, 6, 2)

QR = R - Q
= (2, 7, c) - (0, 6, 5)
= (2, 1, c - 5)

Tegak lurus, PQ . QR = 0
(-a)(2) + (6)(1) + 2(c - 5) = 0
-2a + 6 + 2c - 10 = 0
-2a + 2c - 4 = 0
-2a + 2c = 4
------------------ (÷ -2)
a - c = -2