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Find the angle between the lines whos...

Find the angle between the lines whose direction cosines are connected by the relations `l+m+n=0a n d2//m+2n l-m n=0.`

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Eliminating `n`, we have `(2l+m)(l-m)=0`.
When `2l+m=0`, then `(l)/(1)=(m)/(-2)=(n)/(-2)`.
When `l-m=0`, then `(l)/(1)=(m)/(1)=(n)/(-2)`. Therefore,
Direction ratios are `1, -2, 1 and 1, 1, and -2`
`" "cos theta= (suml_(1)l_(2))/(sqrt((suml_(1)^(2))(suml_(2)^(2))))=-(1)/(2)`
or `" "theta=120^(@)=2pi//3`
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