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                                    If a line makes angles `alpha, beta, gamma` with the coordinate axes, prove that `cos2alpha + cos 2beta + cos2gamma + 1 = 0`. | 
                            
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Answer» `alpha , beta and gamma ` are the angles made by the line with the coordinate axes . ` :. ` We have , `cos^(2) alpha + cos^(2) beta + cos^(2) gamma = 1 ` ….(1) Now, L.H.S. = ` cos 2 alpha + cos 2 beta + cos 2 gamma + 1 ` ` = (2 cos^(2) alpha - 1) + (2 cos^(2) beta -1) + (2 cos^(2) gamma - 1) + 1` ` = 2 ( cos^(2) alpha + cos^(2) beta + cos^(2) gamma) - 3 + 1` ` = 2 * (1) - 2` [from (1)] ` = 2-2` = 0= R.H.S. ` :. cos 2 alpha + cos 2 beta + cos 2 gamma + 1 = 0`.  | 
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