JEE Main 2021 (Online) 27th August Evening ShiftQuestion 1
The angle between the straight lines, whose direction cosines are given by the equations $2l+2m-n=0$ and $mn+nl+lm=0$, is :
3D Geometry
Solution
$n=2l+2m$ in $lm+n(l+m)=0$: $2l^2+5lm+2m^2=0$, $(2l+m)(l+2m)=0$. So direction ratios $(1,-2,-2)$ and $(-2,1,-2)$, dot product $-2-2+4=0$; angle $\frac\pi2$.