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Quadratic Equation and Inequalities question

2022 · 28 Jun · Shift 1 · Q36
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  5. /2022 · 28 Jun · Shift 1 · Q36

Quadratic Equation and Inequalities question

2022 · 28 Jun · Shift 1 · Q36

JEE MainMathematicsQuadratic Equation and InequalitiesNumerical+4 / −1
The number of real solutions of the equation e4x+4e3x−58e2x+4ex+1=0{e^{4x}} + 4{e^{3x}} - 58{e^{2x}} + 4{e^x} + 1 = 0e4x+4e3x−58e2x+4ex+1=0 is ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 2

  1. Substitute to reduce the equation

Let t=ex.t=e^x.t=ex. Since ex>0e^x>0ex>0 for all real xxx, we have t>0.t>0.t>0.

The given equation becomes t4+4t3−58t2+4t+1=0.t^4+4t^3-58t^2+4t+1=0.t4+4t3−58t2+4t+1=0.

  1. Observe the symmetry

The polynomial is palindromic: t4+4t3−58t2+4t+1.t^4+4t^3-58t^2+4t+1.t4+4t3−58t2+4t+1. So we divide by t2t^2t2 (valid since t>0t>0t>0): t2+4t−58+4t+1t2=0.t^2+4t-58+\frac{4}{t}+\frac{1}{t^2}=0.t2+4t−58+t4​+t21​=0.

Rearrange: (t2+1t2)+4(t+1t)−58=0.\left(t^2+\frac{1}{t^2}\right)+4\left(t+\frac{1}{t}\right)-58=0.(t2+t21​)+4(t+t1​)−58=0.

Now use t2+1t2=(t+1t)2−2.t^2+\frac{1}{t^2}=\left(t+\frac{1}{t}\right)^2-2.t2+t21​=(t+t1​)2−2.

Let y=t+1t.y=t+\frac{1}{t}.y=t+t1​. Then the equation becomes y2−2+4y−58=0,y^2-2+4y-58=0,y2−2+4y−58=0, so y2+4y−60=0.y^2+4y-60=0.y2+4y−60=0.

  1. Solve for yyy

y2+4y−60=0y^2+4y-60=0y2+4y−60=0 ⇒(y+10)(y−6)=0.\Rightarrow (y+10)(y-6)=0.⇒(y+10)(y−6)=0. Thus, y=6ory=−10.y=6 \quad \text{or} \quad y=-10.y=6ory=−10.

  1. Use the constraint on yyy

Since t>0t>0t>0, t+1t≥2.t+\frac{1}{t}\ge 2.t+t1​≥2. Hence y≥2y\ge 2y≥2.

So y=−10y=-10y=−10 is impossible, and only y=6y=6y=6 is valid.

  1. Solve for ttt

We now solve t+1t=6.t+\frac{1}{t}=6.t+t1​=6. Multiplying by ttt: t2−6t+1=0.t^2-6t+1=0.t2−6t+1=0.

Thus, t=6±36−42=6±322=3±22.t=\frac{6\pm\sqrt{36-4}}{2}=\frac{6\pm\sqrt{32}}{2}=3\pm 2\sqrt{2}.t=26±36−4​​=26±32​​=3±22​.

Both values are positive, so both are valid values of t=ext=e^xt=ex.

  1. Find the number of real xxx

Since ex=te^x=tex=t, each positive ttt gives exactly one real x=ln⁡tx=\ln tx=lnt.

Therefore there are 222 real solutions.


Comparison with stored answer: Stored correct answer is 222, which matches the derived answer.

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