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Quadratic Equation and Inequalities question

2002 · Shift 0 · Q103
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Quadratic Equation and Inequalities question

2002 · Shift 0 · Q103

JEE MainMathematicsQuadratic Equation and InequalitiesMCQ+4 / −1
If ppp and qqq are the roots of the equation x2+px+q=0,{x^2} + px + q = 0,x2+px+q=0, then
  1. A
    p=1,  q=−2p = 1,\,\,q = - 2p=1,q=−2
  2. B
    p=0,  q=1p = 0,\,\,q = 1p=0,q=1
  3. C
    p=−2,  q=0p = - 2,\,\,q = 0p=−2,q=0
  4. D
    p=−2,  q=1p = - 2,\,\,q = 1p=−2,q=1
View written solutionFree

Correct answer: A

  1. Let the roots of the equation x2+px+q=0x^2+px+q=0x2+px+q=0 be ppp and qqq.

  2. Using Vieta’s formulas for the quadratic x2+px+q=0x^2+px+q=0x2+px+q=0:

    • Sum of roots =−p= -p=−p
    • Product of roots =q= q=q

    Since the roots are themselves ppp and qqq, we get: p+q=−pp+q=-pp+q=−p and pq=qpq=qpq=q

  3. Solve these equations.

    From p+q=−pp+q=-pp+q=−p we get 2p+q=02p+q=02p+q=0 q=−2pq=-2pq=−2p

    From pq=qpq=qpq=q we get q(p−1)=0q(p-1)=0q(p−1)=0

    So either:

    Case 1: q=0q=0q=0

    Then from q=−2pq=-2pq=−2p, 0=−2p⇒p=00=-2p \Rightarrow p=00=−2p⇒p=0 So one solution is (p,q)=(0,0)(p,q)=(0,0)(p,q)=(0,0) which is not among the options.

    Case 2: p=1p=1p=1

    Then from q=−2pq=-2pq=−2p, q=−2(1)=−2q=-2(1)=-2q=−2(1)=−2 So another solution is (p,q)=(1,−2)(p,q)=(1,-2)(p,q)=(1,−2)

  4. Check the options:

    • A: (p,q)=(1,−2)(p,q)=(1,-2)(p,q)=(1,−2) ✓
    • B: (0,1)(0,1)(0,1) ✗
    • C: (−2,0)(-2,0)(−2,0) ✗
    • D: (−2,1)(-2,1)(−2,1) ✗
  5. Hence the correct option is: A\boxed{A}A​

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