# let L1 be the line of intersection of the planes x-2y=5 and x+z=-1and let L2 be the line of intersection of the planes 4x-3y=5 and , z=0  ,find the angle between L1 and L2

Arun Kumar IIT Delhi
8 years ago
L1 will be parallel to
$(1,-1,0)*(1,0,1)=(-1,-1,1)$
L2 will be parallel to
$(4,-3,0)*(0,0,1)=(-3,-4,0)$
we calculate dot product of these two
$cos(\theta)=7/(\sqrt3\sqrt{13})$

Arun Kumar
IIT Delhi