Originally Posted by

**Blizzardy** Hi guys! Need help with this question please,

The plane pi1 has vector EQN: r . (2, 2, 1) = 13. The points P and Q, which are not in pi1, have position vectors 4i + 5j + 7k and 10i + 8j + k respectively, and the line L1, which passes through P and Q, meets pi1 at point R. Find the position vector of R.

The line L2, is contained in pi1, passes through R and is perpendicular to L1. Find a vector which is parallel to L2.

I found OR = (-2, 2, 13)

Since L2 is perpendicular to L1,

Let F be the foot of perpendicular from L2 to L1.

OF lies on L1 => OF = (4, 5, 7) + s(2, 1, -2) for some s E R

OF = (4+2s, 5+s, 7-2s)

RF = (4+2s, 5+s, 7-2s) - (-2, 2, 13) = (6+2s, 3+s, -6-2s)

RF . (2, 1, -2) = 0 (since L1 and L2 are perpendicular)

Solving, s = -3

Then sub. s = -3 into RF which I ended up with a 0?

Any idea why this method doesnt work?