Practicing Success
Unit vectors equally inclined to the vectors $\hat i,\frac{1}{3}(-2\hat i+\hat j+2\hat k), -\frac{1}{5}(4\hat j+3\hat k)$ are |
$±\frac{1}{\sqrt{51}}(\hat i-5\hat j+5\hat k)$ $±\frac{1}{\sqrt{51}}(\hat i-5\hat j-5\hat k)$ $±\frac{1}{\sqrt{51}}(\hat i+5\hat j+5\hat k)$ none of these |
$±\frac{1}{\sqrt{51}}(\hat i-5\hat j+5\hat k)$ |
Let the require unit vector be $\vec b = x\hat i+y\hat j+z\hat k$. It is equally inclined to the given units vectors. Therefore, $(x\hat i+y\hat j+z\hat k). i =\frac{1}{3} (-2\hat i+\hat j+2\hat k). (x\hat i + y\hat j +z\hat k)$ $=-\frac{1}{5}(4\hat j+3\hat k). (x\hat i + y\hat j +3\hat k)$ $⇒x=\frac{1}{3}(-2x+y+2z)=-\frac{1}{5}(4y+ 3z)$ $⇒5x-y-2z=0$ and $5x + 4y + 3z = 0$ $⇒\frac{x}{5}=\frac{y}{-25}=\frac{z}{25}$ $⇒\frac{x}{1}=\frac{y}{-5}=\frac{z}{5}=λ(say) ⇒ x=λ, y=-5λ, z=5λ$ It is given that $\vec r=x\hat i+y\hat j+z\hat k$ is a unit vector. $∴|\vec r|=1⇒x^2 + y^2+z^2=1⇒ λ=±\frac{1}{\sqrt{51}}$ $∴\vec r=±\frac{1}{\sqrt{51}}(\hat i-5\hat j+5\hat k)$ |