If the equation of the line is r=a+λb and the equation of the plane is r⋅n=d. Then, the angle θ between the line and the normal to the plane is cosθ=∣∣∣b∣∣n∣b⋅n∣∣
and so the angle ϕ between the line and the plane is given by 90−θ, i.e., sin(90∘−θ)=cosθ i.e., sinϕ=∣∣∣b∣∣n∣b⋅n∣∣ or ϕ=sin−1∣∣∣b∣∣n∣b⋅n∣∣