Related Formula
θ = | (m₁ - m₂)/(1 + m₁ m₂) |$$\tan \theta = \left| \frac{m_1 - m_2}{1 + m_1 m_2} \right|$$
Core Logic
Find the intercepts of the line 4x + 5y = 20$4x + 5y = 20$ in the first quadrant.
Put y=0 ⇒ x=5$y=0 \Rightarrow x=5$. Point X(5, 0)$X(5, 0)$.
Put x=0 ⇒ y=4$x=0 \Rightarrow y=4$. Point Y(0, 4)$Y(0, 4)$.
The line segment XY$XY$ is trisected by two points, say A$A$ and B$B$.
Point A$A$ divides YX$YX$ in the ratio 2:1$2:1$, and B$B$ divides it in 1:2$1:2$.
Step 1: Trisection Points Calculation
Using the section formula for A$A$ (closer to Y-axis, ratio 1:2 from Y to X):
A = ( (1(5) + 2(0))/(3), (1(0) + 2(4))/(3) ) = ( (5)/(3), (8)/(3) )$$A = \left( \frac{1(5) + 2(0)}{3}, \frac{1(0) + 2(4)}{3} \right) = \left( \frac{5}{3}, \frac{8}{3} \right)$$
Using the section formula for B$B$ (closer to X-axis, ratio 2:1 from Y to X):
B = ( (2(5) + 1(0))/(3), (2(0) + 1(4))/(3) ) = ( (10)/(3), (4)/(3) )$$B = \left( \frac{2(5) + 1(0)}{3}, \frac{2(0) + 1(4)}{3} \right) = \left( \frac{10}{3}, \frac{4}{3} \right)$$
Step 2: Finding Line Slopes
The lines L₁$L_1$ and L₂$L_2$ pass through the origin (0,0)$(0,0)$ to points A$A$ and B$B$.
Slope of OA$OA$ (m₁$m_1$):
m₁ = (8/3 - 0)/(5/3 - 0) = (8)/(5)$$m_1 = \frac{8/3 - 0}{5/3 - 0} = \frac{8}{5}$$
Slope of OB$OB$ (m₂$m_2$):
m₂ = (4/3 - 0)/(10/3 - 0) = (4)/(10) = (2)/(5)$$m_2 = \frac{4/3 - 0}{10/3 - 0} = \frac{4}{10} = \frac{2}{5}$$
Step 3: Calculating Tangent of the Angle
Substitute the slopes into the angle formula:
θ = | (8/5 - 2/5)/(1 + (8/5)(2/5)) |$$\tan \theta = \left| \frac{8/5 - 2/5}{1 + (8/5)(2/5)} \right|$$
θ = (6/5)/(1 + 16/25)$$\tan \theta = \frac{6/5}{1 + 16/25}$$
θ = (6/5)/((25+16)/25) = (6/5)/(41/25)$$\tan \theta = \frac{6/5}{(25+16)/25} = \frac{6/5}{41/25}$$
θ = (6)/(5) × (25)/(41) = (30)/(41)$$\tan \theta = \frac{6}{5} \times \frac{25}{41} = \frac{30}{41}$$
Pattern Recognition
For trisection or specific division of an intercepted segment, identify the axis intercepts first, rapidly apply the internal section formula, compute origin-centered slopes (which equal just the y/x ratio of the points), and pass them into the tan formula.
Chapter Mix
Class 11 Maths: Straight Lines