If l(1), m(1), n(1) and l(2), m(2), n(2)...

If `l_(1), m_(1), n_(1)` and `l_(2), m_(2), n_(2)` are the direction cosines of two vectors and `theta` is the angle between them, then the value of cos `theta` is

`l_(1)l_(2)+m_(1)m_(2)+n_(1)n_(2)`

`l_(1)m_(1)+m_(1)n_(1)+n_(1)l_(1)`

`l_(2)m_(2)+m_(2)n_(2)+n_(2)l_(2)`

`m_(1)l_(2)+l_(2)m_(2)+n_(1)m_(2)`

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