∵a and b are perpendicular to each other. a.b=0 And c is inclined at an angle theta to both a and b. i.e., a. c=b.c=cos‌θ Now, c=αa+βb+γ(a.b) a.c=αa.a+βa.b+γa.(a.b) cos‌θ=α+0+0‌‌‌(∵a.b=0) and b.c=αa.b+βb.b+γ(a.b).b ⇒cos‌θ=β and γ2 may be cos‌2‌θ.