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Quadratic Equation and Inequalities question

2023 · 1 Feb · Shift 2 · Q32
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  5. /2023 · 1 Feb · Shift 2 · Q32

Quadratic Equation and Inequalities question

2023 · 1 Feb · Shift 2 · Q32

JEE MainMathematicsQuadratic Equation and InequalitiesMCQ+4 / −1
The number of integral values of k, for which one root of the equation 2x2−8x+k=02x^2-8x+k=02x2−8x+k=0 lies in the interval (1, 2) and its other root lies in the interval (2, 3), is :
  1. A
    2
  2. B
    0
  3. C
    1
  4. D
    3
View written solutionFree

Correct answer: C

  1. Given quadratic

    2x2−8x+k=02x^2-8x+k=02x2−8x+k=0

    Let its roots be α\alphaα and β\betaβ with α∈(1,2),β∈(2,3).\alpha\in(1,2),\qquad \beta\in(2,3).α∈(1,2),β∈(2,3).

  2. Use sum and product of roots

    For the equation 2x2−8x+k=02x^2-8x+k=02x2−8x+k=0,

    \qquad \alpha\beta=\frac{k}{2}.$$
  3. Use the interval condition with the sum

    Since α∈(1,2)\alpha\in(1,2)α∈(1,2), write 1<α<2.1<\alpha<2.1<α<2. Since β=4−α\beta=4-\alphaβ=4−α, β=4−α.\beta=4-\alpha.β=4−α.

    Now check whether β\betaβ lies in (2,3)(2,3)(2,3):

    • If 1<α<21<\alpha<21<α<2, then 2<4−α<3.2<4-\alpha<3.2<4−α<3. So the second condition is automatically satisfied.

    Hence we only need 1<α<2,1<\alpha<2,1<α<2, where α\alphaα is a root and the other root is 4−α4-\alpha4−α.

  4. Express roots explicitly

    Solve the quadratic: 2x2−8x+k=02x^2-8x+k=02x2−8x+k=0 x2−4x+k2=0x^2-4x+\frac{k}{2}=0x2−4x+2k​=0

    Therefore, x=4±16−2k2=2±16−2k2.x=\frac{4\pm\sqrt{16-2k}}{2}=2\pm\frac{\sqrt{16-2k}}{2}.x=24±16−2k​​=2±216−2k​​.

    So the two roots are

    \qquad 2+\frac{\sqrt{16-2k}}{2}.$$ For one root to be in $(1,2)$ and the other in $(2,3)$, we need $$1<2-\frac{\sqrt{16-2k}}{2}<2$$ and automatically the other will be in $(2,3)$ by symmetry.
  5. Solve the inequality

    From 1<2−16−2k2,1<2-\frac{\sqrt{16-2k}}{2},1<2−216−2k​​, we get −1<−16−2k2-1< -\frac{\sqrt{16-2k}}{2}−1<−216−2k​​ 1>16−2k21>\frac{\sqrt{16-2k}}{2}1>216−2k​​ 2>16−2k.2>\sqrt{16-2k}.2>16−2k​.

    Also, for the smaller root to be less than 222, we need 16−2k>0,\sqrt{16-2k}>0,16−2k​>0, so the roots are distinct and not both equal to 222.

    Thus, 0<16−2k<2.0<\sqrt{16-2k}<2.0<16−2k​<2.

    Squaring, 0<16−2k<4.0<16-2k<4.0<16−2k<4.

    This gives 0<16−2k⇒k<8,0<16-2k \Rightarrow k<8,0<16−2k⇒k<8, and 16−2k<4⇒−2k<−12⇒k>6.16-2k<4 \Rightarrow -2k<-12 \Rightarrow k>6.16−2k<4⇒−2k<−12⇒k>6.

    Hence, 6<k<8.6<k<8.6<k<8.

  6. Integral values of kkk

    The only integer in (6,8)(6,8)(6,8) is k=7.k=7.k=7.

    Therefore, the number of integral values of kkk is 1.1.1.

  7. Option check

    • A: 222 ❌
    • B: 000 ❌
    • C: 111 ✅
    • D: 333 ❌

Final Answer: 1\boxed{1}1​, i.e. Option C.

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