The inclination of a line L (not parallel to the x-axis) is defined as the smallest positive angle measured from the positive direction of the x-axis in a counterclockwise direction to L. Unless otherwise stated the positive direction of L will be considered upward. If L is parallel to the
x-axis its inclination is defined as zero. The slope of a line is defined as the tangent of the angle of inclination. Thus m=tanθ where θ is the inclination and m the slope. The slope of a line passing through two points P_1 (x_1,y_1 ) and P_2 (x_2,y_2 ) is