Question

The direction cosines of the lines which are connected by the relations l – 5m + 3n = 0 and 7l² + 5m² – 3n² = 0 are

The correct answer is

(a)  (1) & (3)

(b)  (1) & (4)

(c)  (2) & (3) 

(d)  (2)& (4)

Answer 1

Correct option  (a) (1) & (3)

Explanation:

Given that l – 5m + 3n = 0 …..(1)

& 7l² + 5m² – 3n² = 0 ……..(2).

Substituting the value of l from (1) in (2),

we get 

7(5m – 3n)² + 5m² – 3n² = 0,

or 180 m² – 210 mn + 60n² = 0

Therefore direction cosines of one line are