Practicing Success
If $\vec a =\hat i +\hat j, \vec b=2\hat j-\hat k$ and $\vec r×\vec a =\vec b ×\vec a, \vec r× \vec b=\vec a×\vec b$ then a unit vector in the direction of $\vec r$ is |
$\frac{1}{\sqrt{11}}(\hat i+3\hat j-\hat k)$ $\frac{1}{\sqrt{11}}(\hat i-3\hat j+\hat k)$ $\frac{1}{\sqrt{3}}(\hat i+\hat j+\hat k)$ none of these |
$\frac{1}{\sqrt{11}}(\hat i+3\hat j-\hat k)$ |
We have, $\vec r×\vec a =\vec b ×\vec a$ and $\vec r× \vec b=\vec a×\vec b$ $⇒\vec r×\vec a =-(\vec r× \vec b)$ $⇒\vec r×(\vec a +\vec b)=0$ $⇒\vec r$ is parallel to $\vec a +\vec b$ $⇒\vec r=λ(\vec a +\vec b)$ $⇒\vec r=λ(\hat i+3\hat j-\hat k)$ $⇒|\vec r|=\sqrt{11}λ$ ∴ Required vector =$\frac{\vec r}{|\vec r|}=\frac{1}{\sqrt{11}}(\hat i+3\hat j-\hat k)$ |