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Quadratic Equation and Inequalities question

2023 · 11 Apr · Shift 2 · Q42
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Quadratic Equation and Inequalities question

2023 · 11 Apr · Shift 2 · Q42

JEE MainMathematicsQuadratic Equation and InequalitiesNumerical+4 / −1
The number of points, where the curve f(x)=e8x−e6x−3e4x−e2x+1,x∈Rf(x)=\mathrm{e}^{8 x}-\mathrm{e}^{6 x}-3 \mathrm{e}^{4 x}-\mathrm{e}^{2 x}+1, x \in \mathbb{R}f(x)=e8x−e6x−3e4x−e2x+1,x∈R cuts xxx-axis, is equal to ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 2

  1. We need the number of real solutions of f(x)=e8x−e6x−3e4x−e2x+1=0.f(x)=e^{8x}-e^{6x}-3e^{4x}-e^{2x}+1=0.f(x)=e8x−e6x−3e4x−e2x+1=0.

  2. Substitute t=e2x.t=e^{2x}.t=e2x. Since e2x>0e^{2x}>0e2x>0 for all real xxx, we have t>0t>0t>0.

    Then the equation becomes t4−t3−3t2−t+1=0.t^4-t^3-3t^2-t+1=0.t4−t3−3t2−t+1=0.

  3. Observe that the polynomial is palindromic: coefficients are 1,−1,−3,−1,11,-1,-3,-1,11,−1,−3,−1,1. Divide by t2t^2t2 (valid since t>0t>0t>0): t2−t−3−1t+1t2=0.t^2-t-3-\frac1t+\frac1{t^2}=0.t2−t−3−t1​+t21​=0.

    Rearranging, (t2+1t2)−(t+1t)−3=0.\left(t^2+\frac1{t^2}\right)-\left(t+\frac1t\right)-3=0.(t2+t21​)−(t+t1​)−3=0.

  4. Let u=t+1t.u=t+\frac1t.u=t+t1​. Then t2+1t2=u2−2.t^2+\frac1{t^2}=u^2-2.t2+t21​=u2−2.

    So the equation becomes (u2−2)−u−3=0,(u^2-2)-u-3=0,(u2−2)−u−3=0, i.e. u2−u−5=0.u^2-u-5=0.u2−u−5=0.

  5. Solve for uuu: u=1±212.u=\frac{1\pm\sqrt{21}}{2}.u=21±21​​.

  6. Since t>0t>0t>0, we know t+1t≥2.t+\frac1t\ge 2.t+t1​≥2. Therefore only u=1+212u=\frac{1+\sqrt{21}}{2}u=21+21​​ is possible, because 1−212<0\frac{1-\sqrt{21}}{2}<021−21​​<0 is not allowed.

  7. Now solve t+1t=1+212.t+\frac1t=\frac{1+\sqrt{21}}{2}.t+t1​=21+21​​. Multiplying by ttt: t2−1+212t+1=0.t^2-\frac{1+\sqrt{21}}{2}t+1=0.t2−21+21​​t+1=0.

    For the number of positive roots in ttt, check discriminant: Δ=(1+212)2−4.\Delta=\left(\frac{1+\sqrt{21}}{2}\right)^2-4.Δ=(21+21​​)2−4.

    Compute:

    =\frac{22+2\sqrt{21}}{4} =\frac{11+\sqrt{21}}{2}.$$ Hence $$\Delta=\frac{11+\sqrt{21}}{2}-4 =\frac{3+\sqrt{21}}{2}>0.$$ So there are two distinct real roots in $t$. Also their product is $1>0$ and sum is positive, hence both roots are positive.
  8. Since t=e2xt=e^{2x}t=e2x is a one-to-one correspondence between x∈Rx\in\mathbb Rx∈R and t>0t>0t>0, each positive root of ttt gives exactly one real xxx.

    Therefore, the curve cuts the xxx-axis at 2 points.

  9. Comparison with stored answer: Stored correct answer = 222. Our derived answer also = 222.

Hence the number of points where the curve cuts the xxx-axis is 2.\boxed{2}.2​.

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