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Q 1860634515.     If `l_1, m_1 , n_1 , l_2 , m_2, n_2` and `l_3 , m_3 , n_3` are the direction cosines of three mutually perpendicular lines, then prove that the line whose direction cosines are proportional to `l_1 + l_2 + l_3 , m_1 + m_2 + m_3` and `n_1 + n_2 + n_3` makes equal angles with them .
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