AP EAMCET Engineering 21 Sep 2020 Shift 1 Paper

Let the direction cosines of line of intersection of the planes x−y=0 and 2x+y+z=0 are a1,b1,c1 so
a1−b1=0
and 2a1+b1+c1=0
By cross-multiplication method, we have
Similarly, let the direction cosines of line of intersection of the planes 2x−z=0 and
x+y−3z=0 are a2,b2,c2 so,
2a2−b2=0
a2+b2−3c2=0
By cross-multiplication method, we have
So angle ′θ′ between lines having directions cosines a1,b1,c1 and a2,b2,c2 respectively . is
=cos−1(0)=90∘