If $\vec{a}$ and $\vec{b}$ are two non-collinear unit vectors, then projection of $\vec{a}+\vec{b}$ on $\vec{a}-\vec{b}$ is equal to:

2 -2 1 None of these

None of these

$(\vec{a}+\vec{b}) . (\vec{a}-\vec{b})=|\vec{a}|^2-|\vec{b}|^2=0$ Thus projection of $\vec{a}+\vec{b}$ on $\vec{a}-\vec{b}$ is zero. Hence (4) is correct answer.