Question

If the line of intersection of the planes ax + by = 3 and ax + by + cz = 0 makes an angle 30° with the plane y - z + 2 = 0, then the direction cosines of line are:

(a) 1/√2 , 0 , 1/2

(b) 1/√2 , -1/√2 , 0

(c) 1/√5 , -2/√5 , 0

(d) 1/2 , -√3/2 , 0

Answer 1

(b) 1/√2 , -1/√2 , 0

Direction ratios of line of intersection (b,-a,0) 

As angle between this line and y-z+2 = 0 is 30°