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NCERT Class XI Mathematics - Straight Lines - Solutions

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Question : 39 of 74
Marks: +1, -0
Find the points on the x-axis, whose distances from the line x3+y4\frac{x}{3}+\frac{y}{4} = 1 are 4 units.
Solution:  
We have an equation of line x3+y4\frac{x}{3}+\frac{y}{4} = 1, which can be written as
4x + 3y – 12 = 0 ... (i)
Let (a, 0) be the point on x-axis whose distance from line (i) is 4 units.
⇒ ∣4×a+3×0−12∣42+32\frac{\left|4\times a+3\times 0-12\right|}{\sqrt{4^{2}+3^{2}}} = 4 ⇒ ∣4a−12∣16+9\frac{\left|4a-12\right|}{\sqrt{16+9}} = 4
⇒ ∣4a−12∣25\frac{\left|4a-12\right|}{\sqrt{25}} = 4 ⇒ ∣4a−12∣5\frac{\left|4a-12\right|}{5} = 4
⇒ |4a – 12| = 20 ⇒ 4a – 12 = ± 20 ⇒ 4a = 12 ± 20 ⇒ a = 3 ± 5
i.e. a = 3 + 5 or a = 3 – 5 ⇒ a = 8 or a = – 2
Hence, the required points on the x-axis are (8, 0) and (–2, 0).
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