Prove that `sin^(−1) (-1/2) + cos^(-1) (-sqrt3/2) = cos^(-1) (-1/2)`
In Δ ABC, prove that, a (b cos C - c cos B) = b2 - c2.
In `triangle ABC` prove that `tan((C-A)/2) = ((c-a)/(c+a))cot B/2`
If a line makes angles α, β, γ with co-ordinate axes, prove that cos 2α + cos2β + cos2γ+ 1 = 0.
If from a point Q (a, b, c) perpendiculars QA and QB are drawn to the YZ and ZX planes respectively, then find the vector equation of the plane QAB.