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STD 11 - 4.2 Quadratic equations and inequations — JEE STD 11 Science — Question

Gujarat BoardEnglish MediumSTD 11 ScienceJEESTD 11 - 4.2 Quadratic equations and inequations4 Marks
MCQ
The equation $e^{4 x}+8 e^{3 x}+13 e^{2 x}-8 e^x+1=0, x \in R$ has:
  • ✓
    two solutions and both are negative
  • B
    no solution
  • C
    four solutions two of which are negative
  • D
    two solutions and only one of them is negative
✓

Answer

Correct option: A.
two solutions and both are negative
a
$e^{4 x}+8 e^{3 x}+13 e^{2 x}-8 e^x+1=0$

Let $e^x=t$

Now, $t^4+8 t^5+13 t^2-8 t+1=0$

Dividing equation by $t ^2$,

$t^2+8 t+13-\frac{8}{t}+\frac{1}{t^2}=0$

$t^2+\frac{1}{t^2}+8\left(t-\frac{1}{t}\right)+13=0$

$\left(t-\frac{1}{t}\right)^2+2+8\left(t-\frac{1}{t}\right)+13=0$

Let $t-\frac{1}{t}=z$

$z^2+8 z+15=0$

$(z+3)(z+5)=0$

$z=-3 \text { or } z=-5$

So, $t -\frac{1}{ t }=-3$ or $t -\frac{1}{ t }=-5$

$t^2+3 t-1=0 \text { or } t^2+5 t-1=0$

$t=\frac{-3 \pm \sqrt{13}}{2} \text { or } t =\frac{-5 \pm \sqrt{29}}{2}$

as $t = e ^{ x }$ so $t$ must be positive,

$t=\frac{\sqrt{13}-3}{2} \text { or } \frac{\sqrt{29}-5}{2}$

So, $x=\ln \left(\frac{\sqrt{13}-3}{2}\right)$ or $x=\ln \left(\frac{\sqrt{29}-5}{2}\right)$

Hence two solution and both are negative.

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