The length of the projections of AB on the coordinate planes are d1,d2 and d3 Now, AB=d (cos‌α‌i+cos‌β‌j+cos‌γ‌k)=(d‌cos‌α‌i+d‌cos‌β‌j+d‌cos‌γ‌k) Let θ be the angle between AB and the XY plane. Then, d=d‌cos‌θ Now, sin‌θ=