Bro, in the denominator after cos ^2 ϴ step
you wrote k^2(a^2 + b^2+c^2) as k^2(a+b+c)^2
ab+bc+ca = 0 is not a given condition. We have to prove that it is zero right?
Ok bro, I tried the same calculation as yours with the above correction. We get the condition ab+bc+ca =0 Thanks.
Also please do answer why sum of squares of l m and n is not equal to 1. thought they are direction cosines?