Angle between two straight lines

Two straight lines in a plane would either be parallel or coincide or intersect. Normally when two straight lines intersect, they form two angles at the point of intersection. One is an acute angle and another is an obtuse angle or equal. Both these angles would be supplements(Sum equals ◦ ) of each other.

By definition, when we say ‘angle between two straight lines’ we mean the acute angle between the two lines. Let y = m x + c and y = m x + c be the equations of two straight lines and let these two lines make angles θ and θ with x - axis. Then m = tan θ and m = tan θ If φ (phi) is the angle between these two straight lines, then φ = θ − θ ⇒ tan φ = tan ( θ − θ ) ⇒ tan φ = m − m + m m ⇒ φ = tan − m − m + m m θ φ θ