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** Q 2657145984. ** lf `l_1, m_1 , n_1` and `I_2, m_2, n_2` are the direction cosines

of two mutually perpendicular lines, show that

the direction cosines of the line perpendicular

to both of these are `m_1 n_2 - m_2 n_1 , n_1 I_2 - n_2 I_1,

I_1 m_2 -I_2 m_1`