Slope of a Line and Angle between Two Lines

Tips :

• If the inclination of a non-vertical line is ‘θ’ then tan θ is called slope of the line and is usually denoted by m, thus m = tan θ
• Slope of a horizontal line is ‘0’ [\because θ = 0°]
• Slope of a vertical line is not defined [\because θ = 90°]
• Slope of the line joining two points A(x₁, y₁), B (x₂ y₂) is \tt m=\frac{y_{2}-y_{1}}{x_{2}-x_{1}}
• Two non-vertical lines are parallel if their slopes are equal.
• Two non-vertical lines are perpendicular if their product of slopes is ‘−1’
• If ‘3’ points are collinear (A, B, C) then slope of AB = Slope of BC = Slope of AC (or) Area ΔABC = 0 (or) one of the points lies on the line joining of the other two points (or) sum of two of the distances AB, BC, CA, is equal to the third
• If ‘θ’ is an acute angle between the lines having slopes m₁ and m₂ then \tt \tan \theta=\begin{vmatrix}\frac{m_{1}-m_{2}}{1+m_{1}m_{2}} \end{vmatrix}
• If ‘θ’ is an acute angle between the lines a₁x + b₁y + c₁ = 0 a₂ x + b₂ y + c₂ = 0 then \tt cos \theta=\begin{vmatrix}\frac{a_{1}a_{2}+b_{1}b_{2}}{\sqrt{a^{2}_{1} + b^{2}}_{1} .\sqrt{{a_2^2}+b_2^2}}\end{vmatrix}and \ \tan\theta=\begin{vmatrix}\frac{a_{1}b_{2}-a_{2}b_{1}}{a_{1}a_{2}+b_{1}b_{2}}\end{vmatrix} other angle between the lines is π – θ.
• If a₁ x + b₁ y + c₁ = 0 and a₂ x + b₂ y + c₂ = 0 are parallel to y–axis then angle between them is ‘o’ or ‘π’.
• If any one of the lines is parallel to y-axis and other makes on angle ‘θ’ with positive direction of x-axis then angle between lines is |90° – θ|.

Tricks :

• If a line is equally inclined to the axes, then it will make an angle of 45° or 135° with x–axis (Positive direction of x-axis) and hence its slope will be tan 45° or tan 135° = ± 1.
• Slope of the line ax + by + c = 0, b ≠ 0 is \tt \frac{-a}{b}