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Straight Lines and Pair of Straight Lines question

2022 · 29 Jun · Shift 1 · Q29
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Straight Lines and Pair of Straight Lines question

2022 · 29 Jun · Shift 1 · Q29

JEE MainMathematicsStraight Lines and Pair of Straight LinesMCQ+4 / −1
The distance between the two points A and A' which lie on y = 2 such that both the line segments AB and A' B (where B is the point (2, 3)) subtend angle π4{\pi \over 4}4π​ at the origin, is equal to :
  1. A
    10
  2. B
    485{48 \over 5}548​
  3. C
    525{52 \over 5}552​
  4. D
    3
View written solutionFree

Correct answer: C

  1. Set up the geometry

Let the required points on the line y=2y=2y=2 be A(x,2),A′(x′,2).A(x,2), \qquad A'(x',2).A(x,2),A′(x′,2). Given point: B=(2,3),O=(0,0).B=(2,3), \qquad O=(0,0).B=(2,3),O=(0,0).

We are told that the line segments ABABAB and A′BA'BA′B each subtend an angle π4\frac{\pi}{4}4π​ at the origin.
That means the angle between the lines OAOAOA and OBOBOB is π4\frac{\pi}{4}4π​, and similarly for OA′OA'OA′ and OBOBOB.

So we need all points A(x,2)A(x,2)A(x,2) such that ∠AOB=π4.\angle AOB=\frac{\pi}{4}.∠AOB=4π​.


  1. Use slope formula for angle between two lines

Slope of OBOBOB: m1=32.m_1=\frac{3}{2}.m1​=23​.

Slope of OAOAOA: m2=2x.m_2=\frac{2}{x}.m2​=x2​.

Angle between two lines with slopes m1,m2m_1,m_2m1​,m2​ is given by tan⁡θ=∣m2−m11+m1m2∣.\tan\theta=\left|\frac{m_2-m_1}{1+m_1m_2}\right|.tanθ=​1+m1​m2​m2​−m1​​​.

Here θ=π4\theta=\frac{\pi}{4}θ=4π​, so tan⁡θ=1\tan\theta=1tanθ=1. Therefore, ∣2x−321+32⋅2x∣=1.\left|\frac{\frac{2}{x}-\frac{3}{2}}{1+\frac{3}{2}\cdot\frac{2}{x}}\right|=1.​1+23​⋅x2​x2​−23​​​=1.

Simplify: ∣2x−321+3x∣=1.\left|\frac{\frac{2}{x}-\frac{3}{2}}{1+\frac{3}{x}}\right|=1.​1+x3​x2​−23​​​=1.

Multiply numerator and denominator by 2x2x2x: ∣4−3x2x+6∣=1.\left|\frac{4-3x}{2x+6}\right|=1.​2x+64−3x​​=1.

So, ∣4−3x∣=∣2x+6∣.|4-3x|=|2x+6|.∣4−3x∣=∣2x+6∣.


  1. Solve the equation

This gives two cases:

Case 1

4−3x=2x+64-3x=2x+64−3x=2x+6 −5x=2-5x=2−5x=2 x=−25.x=-\frac{2}{5}.x=−52​.

Case 2

4−3x=−(2x+6)4-3x=-(2x+6)4−3x=−(2x+6) 4−3x=−2x−64-3x=-2x-64−3x=−2x−6 x=10.x=10.x=10.

Thus the two points are A(−25,2),A′(10,2).A\left(-\frac{2}{5},2\right), \qquad A'(10,2).A(−52​,2),A′(10,2).


  1. Find the distance between AAA and A′A'A′

Since both points lie on the horizontal line y=2y=2y=2, their distance is just the difference of their xxx-coordinates: AA′=∣10−(−25)∣=10+25=525.AA'=\left|10-\left(-\frac{2}{5}\right)\right|=10+\frac{2}{5}=\frac{52}{5}.AA′=​10−(−52​)​=10+52​=552​.


  1. Compare with options

525\frac{52}{5}552​ corresponds to Option C.


  1. Verification with stored answer

Stored correct answer: C
Derived answer: C

They agree.

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