Find the acute angle between the two straight lines whose direction cosines are given by `l+m+n=0` and `l^2+m^2-n^2=0`

Explanation

To find the acute angle between the two straight lines whose direction cosines are given by the equations \( l + m + n = 0 \) and \( l^2 + m^2 - n^2 = 0 \), we can follow these steps:

### Step 1: Express \( n \) in terms of \( l \) and \( m \)
From the first equation, we can express \( n \):
\[ n = -l - m \]

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