We have the equations r² = x² + y², x = r cos(θ), and y = r sin(θ) that can be used to solve this problem. In this case we only need the second two equations ( x = r cos(θ), and y = r sin(θ) ) as we don't need to apply the concept of circles etc here.
Given : x = - 9,
( Substitute r cos(θ) for x )
r cos(θ) = - 9,
r = - 9 / cos(θ)
( Remember that sec is the reciprocal of cos(θ). Substitute sec for 1 / cos(θ) )
r = - 9 sec(θ)
Therefore the third option is the correct solution.
