......(i) where, α,β,γ are the angles made by a line with positive direction of the axes of reference. We know that, I2+m2+n2=1 ∴ cos2α+cos2β+cos2γ=1.....(ii)[I=cos‌α,m=cos‌β‌and‌n=cos‌γ] ⇒ (1−sin2α)+(1−sin2β)+(1−sin2γ)=1 ⇒ 3−(sin2α+sin2β+sin2γ)=1 ⇒ sin2α+sin2β+sin2γ=2 ....(iii) cos‌θ=