With respect to a fixed origin O, the line l1 has vector equation

r = (-9x, 8y, 5z) + u(5x, -4y, -3z)

where u is a scalar parameter

The point A is on l1 where u=2

(a) write down the coordinates of A (I got this one)

The acute angle between OA and l1 is θ, where O is the origin

(b)Find the value of cosθ

The point B such that OB=3OA

The line l2 passes through point B and is parallel to l1

(c) Find a vector equation for l2

(d)Find the length of OB, as a surd

The pount X lies on l2. Given that the vector OX is perpendicular to l2,

(e) find the length of OX, giving your answer to 3 s f.

question has total of 10 marks