Practicing Success
If $\vec a,\vec b,\vec c$ and $\vec d$ are unit vectors such that $(\vec a×\vec b). (\vec c×\vec d) = 1$ and $\vec a.\vec c=\frac{1}{2}$, then |
$\vec a,\vec b,\vec c$ are non-coplanar $\vec b,\vec c,\vec d$ are non-coplanar $\vec b,\vec d$ are non-parallel $\vec a, \vec d$ are parallel and $\vec b, \vec c$ are parallel |
$\vec b,\vec d$ are non-parallel |
Since $\vec a,\vec b,\vec c$ and $\vec d$ are unit vectors. $∴\vec a×\vec b=|\vec a||\vec b|\sin α\hat{n_1}$, where $\hat{n_1}$ is a unit vector perpendicular to the plane of $\vec a$ and $\vec b$ and, $\vec c×\vec d=|\vec c||\vec d|\sin β\hat{n_2}$, where $\hat{n_2}$ is a unit vector perpendicular to the plane of $\vec a$ and $\vec b$ $∴(\vec a×\vec b).(\vec c×\vec d)=1$ $⇒\sin α\,\sin β(\hat{n_1}.\hat{n_2})=1$ $⇒\sin α\,\sin β\cos θ=1$, where θ is the angle between $\hat{n_1}$ and $\hat{n_2}$ $⇒\sin α=1,\sin β=1,\cos θ=1$ $[∵α ∈ [0, π], β∈ [0, π]]$ $⇒α=π/2,β=π/2$ and $θ=0$ Now, $\vec a.\vec c=\frac{1}{2}$ $⇒\cos γ=\frac{1}{2}$, where $γ$ is the angle between $\vec a$ and $\vec c$. $⇒γ=\frac{π}{3}$ We have, $θ=0$ $∴\hat{n_1}||\hat{n_2}⇒\vec a×\vec b||\vec c×\vec d⇒\vec a,\vec b,\vec c$ are non-coplanar Thus, we have $\vec a⊥\vec b, \vec c⊥\vec d$ and angle between $\vec a$ and $\vec c$ is $π/3$. So, we have the following possibilities. In both the cases, we find that $\vec b$ is non-parallel to $\vec d$. Hence, option (3) is correct. |