The acute angle α between two straight lines having slopes m1 and m2, is given by; tan α=(m1−m21+m1m2∣∣
Let L1 and L2 be two non-vertical lines with slopes m1 and m2 respectively. If α1 and α2 are the inclinations of lines L1 and L2, respectively. the acute angle (say θ) between lines L1 and L2 , is given by