Angle between two straight lines is given by \[\frac{m-n}{1+mn} \]
where m and n are the slopes of straight line 1 and line 2 respectively.
case1: if m*n = -1 then lines are perpendicular
case 2: if m=n lines are parellel.
case 3. \[m\neq n\neq -1 \] lines are intersecting.
for examples
y=2x+3 and y= x+2
here m=2 n=1
lines are intersecting
y=2x+8 y=-(1/2)x+6
m=2 n=-1/2 here m*n=-1 so lines are perpendicular
