If `theta` is the acute angle between the lines with slopes m1 and m2 then `tantheta = (m_1 -m_2)/(1+m_1m_2)`.

Explanation

Let the slope of two line be


`tan a_1 = m_1->(1)` and `tan a_2 = m_2->(2)`


now `theta = theta_2-theta_1`


Now, `tan theta = tan (theta_2-theta_1) `
`=>tan theta
= (tan theta_2 – tan theta_1 ) / (1- tan theta_1tan theta_2)`
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