Angle between two lines
<p>I need to find angle between two line in my c# program. Can someone help me to calculate the points if only start end poinst given.</p>

<p><a href="https://i.stack.imgur.com/eTPIJ.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/eTPIJ.png" alt="enter image description here"></a> 
for line L1 starting point(X1,Y1) and End point (X2,Y2) Given same as for L2 also</p>