Question:medium

The slope of a line is double of the slope of another line. If tangent of the angle between them is \(\frac{1}{3}\) , find the slopes of he lines.

Updated On: Jan 21, 2026
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Solution and Explanation

Given:

Let the slopes of the two lines be m1 and m2.

It is given that:

m1 = 2m2

The tangent of the angle θ between the two lines is:

tan θ = 1/3


Step 1: Use the formula for angle between two lines

The tangent of the angle between two lines with slopes m1 and m2 is:

tan θ = |(m1 − m2) / (1 + m1m2)|


Step 2: Substitute given values

Substitute m1 = 2m2:

tan θ = |(2m2 − m2) / (1 + 2m22)|

tan θ = |m2 / (1 + 2m22)|

Given tan θ = 1/3:

|m2 / (1 + 2m22)| = 1/3


Step 3: Solve for m2

Cross-multiplying:

3|m2| = 1 + 2m22

2m22 − 3m2 + 1 = 0

Solving the quadratic equation:

2m22 − 3m2 + 1 = 0

(2m2 − 1)(m2 − 1) = 0

m2 = 1 or 1/2


Step 4: Find m1

m1 = 2m2

If m2 = 1, then m1 = 2

If m2 = 1/2, then m1 = 1


Final Answer:

The possible pairs of slopes of the two lines are:

(m1, m2) = (2, 1)   or   (1, 1/2)

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