Angle Between Lines Drawn Through Rectangle

Given a rectangle with two straight lines drawn through, we can find the angle between the lines. If the gradients of the lines are  
\[m_1\]
  and  
\[m_2\]
  with  
\[m_1 \gt m_2\]
  then the angle between the lines is  
\[tan^{-1} (m_1) - tan^{-1}(m_2)\]
.

angles between lines drawn through a rectangle

The line  
\[l_1\]
  goes along 7+3=10 and up 6-3=3. The gradient is  
\[\frac{3}{10}\]
.
The line  
\[l_2\]
  goes along 3-6=-3 and up 3+5=8. The gradient is  
\[\frac{8}{-3}\]
.
The angle  
\[\alpha\]
  is  
\[tan^{-1}( \frac{8}{-})- tan^{-1}(\frac{3}{10})=93.9^o\]
  to 1 decimal place.

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