Practicing Success
The number of distinct real values of λ for which the vectors $\vec a=λ^3\hat i+\hat k,\vec b=\hat i-λ^3\hat j$ and $\vec c=\hat i(2λ-\sin λ)\hat j-λ\hat k$ are coplanar is |
0 1 2 3 |
1 |
For $\vec a,\vec b,\vec c$ to be coplanar, we must have $[\vec a\,\vec b\,\vec c]=0$ $⇒\begin{vmatrix}1&0&1\\1&-λ^3&0\\1&2λ-\sin λ&-λ\end{vmatrix}=0$ $⇒λ^7+λ^3+2λ=\sin λ$ This is true for $λ = 0$. For non-zero values of λ it gives $λ^6+λ^2+2=\frac{\sin λ}{λ}$ ...(i) We know that $\frac{\sin x}{x}<1$ for all $x≠ 0$. Therefore, LHS of (i) is greater than 2 and RHS is less than 1. So, (i) is not true for any non-zero λ. Hence, there is only one value of λ. |