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Application of Derivatives question

2025 · 22 Jan · Shift 2 · Q40
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  5. /2025 · 22 Jan · Shift 2 · Q40

Application of Derivatives question

2025 · 22 Jan · Shift 2 · Q40

JEE MainMathematicsApplication of DerivativesMCQ+4 / −1
Let f(x)=∫0x2t2−8t+15etdt,x∈Rf(x)=\int_0^{x^2} \frac{\mathrm{t}^2-8 \mathrm{t}+15}{\mathrm{e}^{\mathrm{t}}} \mathrm{dt}, x \in \mathbf{R}f(x)=∫0x2​ett2−8t+15​dt,x∈R. Then the numbers of local maximum and local minimum points of fff, respectively, are :
  1. A
    3 and 2
  2. B
    2 and 2
  3. C
    2 and 3
  4. D
    1 and 3
View written solutionFree

Correct answer: C

  1. Given function
f(x)=∫0x2t2−8t+15et dt=∫0x2(t2−8t+15)e−t dtf(x)=\int_0^{x^2} \frac{t^2-8t+15}{e^t}\,dt =\int_0^{x^2} (t^2-8t+15)e^{-t}\,dtf(x)=∫0x2​ett2−8t+15​dt=∫0x2​(t2−8t+15)e−tdt

We need the number of local maxima and local minima points of f(x)f(x)f(x).


  1. Differentiate using Fundamental Theorem of Calculus and chain rule

Let

g(t)=t2−8t+15et=(t2−8t+15)e−tg(t)=\frac{t^2-8t+15}{e^t}=(t^2-8t+15)e^{-t}g(t)=ett2−8t+15​=(t2−8t+15)e−t

Then

f′(x)=g(x2)⋅ddx(x2)f'(x)=g(x^2)\cdot \frac{d}{dx}(x^2)f′(x)=g(x2)⋅dxd​(x2)

So,

f′(x)=2x⋅x4−8x2+15ex2f'(x)=2x\cdot \frac{x^4-8x^2+15}{e^{x^2}}f′(x)=2x⋅ex2x4−8x2+15​

Factor the polynomial:

x4−8x2+15=(x2−3)(x2−5)x^4-8x^2+15=(x^2-3)(x^2-5)x4−8x2+15=(x2−3)(x2−5)

Hence

f′(x)=2x(x2−3)(x2−5)ex2f'(x)=\frac{2x(x^2-3)(x^2-5)}{e^{x^2}}f′(x)=ex22x(x2−3)(x2−5)​

Since ex2>0e^{x^2}>0ex2>0 for all xxx, the sign of f′(x)f'(x)f′(x) depends only on

2x(x2−3)(x2−5)2x(x^2-3)(x^2-5)2x(x2−3)(x2−5)

Critical points are:

x=0, ±3, ±5x=0,\ \pm\sqrt{3},\ \pm\sqrt{5}x=0, ±3​, ±5​
  1. Sign analysis of f′(x)f'(x)f′(x)

We check the intervals:

  • (−∞,−5)(-\infty,-\sqrt5)(−∞,−5​)
  • (−5,−3)(-\sqrt5,-\sqrt3)(−5​,−3​)
  • (−3,0)(-\sqrt3,0)(−3​,0)
  • (0,3)(0,\sqrt3)(0,3​)
  • (3,5)(\sqrt3,\sqrt5)(3​,5​)
  • (5,∞)(\sqrt5,\infty)(5​,∞)

Take signs of factors:

(i) x<−5x< -\sqrt5x<−5​

  • x<0x<0x<0 so 2x<02x<02x<0
  • x2−3>0x^2-3>0x2−3>0
  • x2−5>0x^2-5>0x2−5>0

Thus f′(x)<0f'(x)<0f′(x)<0.

(ii) −5<x<−3-\sqrt5<x<-\sqrt3−5​<x<−3​

  • 2x<02x<02x<0
  • x2−3>0x^2-3>0x2−3>0
  • x2−5<0x^2-5<0x2−5<0

Thus f′(x)>0f'(x)>0f′(x)>0.

(iii) −3<x<0-\sqrt3<x<0−3​<x<0

  • 2x<02x<02x<0
  • x2−3<0x^2-3<0x2−3<0
  • x2−5<0x^2-5<0x2−5<0

Thus f′(x)<0f'(x)<0f′(x)<0.

(iv) 0<x<30<x<\sqrt30<x<3​

  • 2x>02x>02x>0
  • x2−3<0x^2-3<0x2−3<0
  • x2−5<0x^2-5<0x2−5<0

Thus f′(x)>0f'(x)>0f′(x)>0.

(v) 3<x<5\sqrt3<x<\sqrt53​<x<5​

  • 2x>02x>02x>0
  • x2−3>0x^2-3>0x2−3>0
  • x2−5<0x^2-5<0x2−5<0

Thus f′(x)<0f'(x)<0f′(x)<0.

(vi) x>5x>\sqrt5x>5​

  • 2x>02x>02x>0
  • x2−3>0x^2-3>0x2−3>0
  • x2−5>0x^2-5>0x2−5>0

Thus f′(x)>0f'(x)>0f′(x)>0.

So the sign chart is:

−, +, −, +, −, +-,\ +,\ -,\ +,\ -,\ +−, +, −, +, −, +

across the critical points

−5, −3, 0, 3, 5-\sqrt5,\ -\sqrt3,\ 0,\ \sqrt3,\ \sqrt5−5​, −3​, 0, 3​, 5​
  1. Determine local maxima and minima

A local minimum occurs when f′(x)f'(x)f′(x) changes from negative to positive.

A local maximum occurs when f′(x)f'(x)f′(x) changes from positive to negative.

Now check each critical point:

  • At x=−5x=-\sqrt5x=−5​: f′f'f′ changes −→+- \to +−→+
    ⇒\Rightarrow⇒ local minimum

  • At x=−3x=-\sqrt3x=−3​: f′f'f′ changes +→−+ \to -+→−
    ⇒\Rightarrow⇒ local maximum

  • At x=0x=0x=0: f′f'f′ changes −→+- \to +−→+
    ⇒\Rightarrow⇒ local minimum

  • At x=3x=\sqrt3x=3​: f′f'f′ changes +→−+ \to -+→−
    ⇒\Rightarrow⇒ local maximum

  • At x=5x=\sqrt5x=5​: f′f'f′ changes −→+- \to +−→+
    ⇒\Rightarrow⇒ local minimum

Therefore,

  • Number of local maxima =2=2=2
  • Number of local minima =3=3=3

  1. Compare with options

This corresponds to:

Option C: 2 and 3\boxed{\text{Option C: } 2 \text{ and } 3}Option C: 2 and 3​
  1. Comparison with stored correct answer

Stored correct answer is C, which matches our result.

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