Practicing Success
If \(\vec{a}\) is a unit vector and $(\vec{x}-\vec{a}) \cdot(\vec{x}+\vec{a})=8$ then \(|\vec{x}|\) is: |
2 3 ±3 5 |
3 |
Given $(\vec{x}-\vec{a}) \cdot(\vec{x}+\vec{a})=8$ & $\vec{a}$ is a unit vector which means $|\vec{a}|=1$ So, $(\vec{x}-\vec{a})(\vec{x}+\vec{a})⇒|\vec{x}|^2-|\vec{a}|^2$ $⇒|\vec{x}|^2-|\vec{a}|^2=8$ $⇒|\vec{x}|^2-1=8⇒|\vec{x}|^2=8+1⇒9$ $∴ |\vec{x}|$ is positive, so $|\vec{x}|=3$ Option 2 is correct. |