If a line makes angles α, β, γ with positive direction of the coordinate axes, then $\mathit{c}\mathit{o}{\mathit{s}}^2\mathit{\alpha }+\mathit{c}\mathit{o}{\mathit{s}}^2\mathit{\beta }+\mathit{c}\mathit{o}{\mathit{s}}^2\mathit{\gamma }=1$
$\Rightarrow \left(1-\mathit{s}\mathit{i}{\mathit{n}}^2\mathit{\alpha }\right)+\left(1-\mathit{s}\mathit{i}{\mathit{n}}^2\mathit{\beta }\right)+\left(1-\mathit{s}\mathit{i}{\mathit{n}}^2\mathit{\gamma }\right)=1$
$\Rightarrow \mathit{s}\mathit{i}{\mathit{n}}^2\mathit{\alpha }+\mathit{s}\mathit{i}{\mathit{n}}^2\mathit{\beta }+\mathit{s}\mathit{i}{\mathit{n}}^2\mathit{\gamma }=2$
Both (A) and (R) are true and (R) is the correct explanation of (A).