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Straight Lines and Pair of Straight Lines question

2009 · Shift 0 · Q40
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Straight Lines and Pair of Straight Lines question

2009 · Shift 0 · Q40

JEE MainMathematicsStraight Lines and Pair of Straight LinesMCQ+4 / −1
The lines p(p2+1)x−y+q=0p\left( {{p^2} + 1} \right)x - y + q = 0p(p2+1)x−y+q=0 and (p2+1)2x+(p2+1)y+2q\left( {{p^2} + 1} \right){}^2x + \left( {{p^2} + 1} \right)y + 2q(p2+1)2x+(p2+1)y+2q =0=0=0 are perpendicular to a common line for :
  1. A
    exactly one values of ppp
  2. B
    exactly two values of ppp
  3. C
    more than two values of ppp
  4. D
    no value of ppp
View written solutionFree

Correct answer: A

  1. Write each line in slope form

Given lines: p(p2+1)x−y+q=0p(p^2+1)x-y+q=0p(p2+1)x−y+q=0 and (p2+1)2x+(p2+1)y+2q=0(p^2+1)^2x+(p^2+1)y+2q=0(p2+1)2x+(p2+1)y+2q=0

We find their slopes.


  1. Slope of the first line

From p(p2+1)x−y+q=0p(p^2+1)x-y+q=0p(p2+1)x−y+q=0 we get −y=−p(p2+1)x−q-y=-p(p^2+1)x-q−y=−p(p2+1)x−q y=p(p2+1)x+qy=p(p^2+1)x+qy=p(p2+1)x+q

So slope of first line is m1=p(p2+1)m_1=p(p^2+1)m1​=p(p2+1)


  1. Slope of the second line

From (p2+1)2x+(p2+1)y+2q=0(p^2+1)^2x+(p^2+1)y+2q=0(p2+1)2x+(p2+1)y+2q=0 we get (p2+1)y=−(p2+1)2x−2q(p^2+1)y=-(p^2+1)^2x-2q(p2+1)y=−(p2+1)2x−2q y=−(p2+1)x−2qp2+1y=-(p^2+1)x-\frac{2q}{p^2+1}y=−(p2+1)x−p2+12q​

So slope of second line is m2=−(p2+1)m_2=-(p^2+1)m2​=−(p2+1)


  1. Condition for both lines to be perpendicular to a common line

If two lines are both perpendicular to the same line, then they must be parallel to each other.

Hence we need: m1=m2m_1=m_2m1​=m2​

So, p(p2+1)=−(p2+1)p(p^2+1)=-(p^2+1)p(p2+1)=−(p2+1)

Since p2+1>0p^2+1>0p2+1>0 for all real ppp, we can divide by (p2+1)(p^2+1)(p2+1): p=−1p=-1p=−1

Thus there is exactly one real value of ppp.


  1. Check options
  • A: exactly one value of ppp ✔️
  • B: exactly two values of ppp ✘
  • C: more than two values of ppp ✘
  • D: no value of ppp ✘

Therefore, the correct option is: A\boxed{\text{A}}A​

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