Prove that 1,1,1 cannot be direction cosines of a straight line.

Explanation

Sum of squares of direction cosines of a straight line is always `1`.

Here, sum of squares of direction cosines ` = 1^2+1^2+1^2 = 3 `

As `3 !=1`, so `(1,1,1)` can not be direction cosines of a straight line.