Skip to main content
Back to Application of Derivatives
JEE Main 2023
Application of Derivatives
Application of Derivatives
Easy

Question

Let f(x)=(x+3)2(x2)3,x[4,4]f(x)=(x+3)^2(x-2)^3, x \in[-4,4]. If MM and mm are the maximum and minimum values of ff, respectively in [4,4][-4,4], then the value of MmM-m is

Options

Solution

Key Concepts and Formulas

  • Extreme Value Theorem: A continuous function on a closed interval [a,b][a, b] attains both an absolute maximum and an absolute minimum on that interval.
  • Critical Points: Points where the derivative of a function is zero or undefined.
  • Product Rule: ddx(uv)=uv+uv\frac{d}{dx}(uv) = u'v + uv'

Step-by-Step Solution

Step 1: Find the derivative of f(x)f(x)

We are given the function f(x)=(x+3)2(x2)3f(x) = (x+3)^2(x-2)^3 and we want to find its derivative, f(x)f'(x), to locate critical points. We will use the product rule, where u=(x+3)2u = (x+3)^2 and v=(x2)3v = (x-2)^3.

u=ddx(x+3)2=2(x+3)u' = \frac{d}{dx}(x+3)^2 = 2(x+3) v=ddx(x2)3=3(x2)2v' = \frac{d}{dx}(x-2)^3 = 3(x-2)^2

Applying the product rule: f(x)=uv+uv=2(x+3)(x2)3+(x+3)23(x2)2f'(x) = u'v + uv' = 2(x+3)(x-2)^3 + (x+3)^2 \cdot 3(x-2)^2

Now, we factor out the common terms (x+3)(x+3) and (x2)2(x-2)^2: f(x)=(x+3)(x2)2[2(x2)+3(x+3)]f'(x) = (x+3)(x-2)^2[2(x-2) + 3(x+3)] f(x)=(x+3)(x2)2[2x4+3x+9]f'(x) = (x+3)(x-2)^2[2x - 4 + 3x + 9] f(x)=(x+3)(x2)2[5x+5]f'(x) = (x+3)(x-2)^2[5x + 5] f(x)=5(x+3)(x2)2(x+1)f'(x) = 5(x+3)(x-2)^2(x+1)

The derivative is f(x)=5(x+3)(x2)2(x+1)f'(x) = 5(x+3)(x-2)^2(x+1).

Step 2: Find the critical points

Critical points occur where f(x)=0f'(x) = 0 or f(x)f'(x) is undefined. Since f(x)f'(x) is a polynomial, it is defined for all real numbers. Thus, we only need to solve f(x)=0f'(x) = 0:

5(x+3)(x2)2(x+1)=05(x+3)(x-2)^2(x+1) = 0

This gives us the following critical points:

  • x+3=0    x=3x+3 = 0 \implies x = -3
  • (x2)2=0    x=2(x-2)^2 = 0 \implies x = 2
  • x+1=0    x=1x+1 = 0 \implies x = -1

The critical points are x=3,1,2x = -3, -1, 2. All these points lie within the interval [4,4][-4, 4].

Step 3: Evaluate f(x)f(x) at the critical points and endpoints

We need to evaluate f(x)=(x+3)2(x2)3f(x) = (x+3)^2(x-2)^3 at the critical points x=3,1,2x = -3, -1, 2 and the endpoints x=4,4x = -4, 4.

  1. f(4)=(4+3)2(42)3=(1)2(6)3=1(216)=216f(-4) = (-4+3)^2(-4-2)^3 = (-1)^2(-6)^3 = 1 \cdot (-216) = -216
  2. f(3)=(3+3)2(32)3=(0)2(5)3=0f(-3) = (-3+3)^2(-3-2)^3 = (0)^2(-5)^3 = 0
  3. f(1)=(1+3)2(12)3=(2)2(3)3=4(27)=108f(-1) = (-1+3)^2(-1-2)^3 = (2)^2(-3)^3 = 4 \cdot (-27) = -108
  4. f(2)=(2+3)2(22)3=(5)2(0)3=250=0f(2) = (2+3)^2(2-2)^3 = (5)^2(0)^3 = 25 \cdot 0 = 0
  5. f(4)=(4+3)2(42)3=(7)2(2)3=498=392f(4) = (4+3)^2(4-2)^3 = (7)^2(2)^3 = 49 \cdot 8 = 392

So we have the following values: f(4)=216f(-4) = -216, f(3)=0f(-3) = 0, f(1)=108f(-1) = -108, f(2)=0f(2) = 0, and f(4)=392f(4) = 392.

Step 4: Identify the maximum (MM) and minimum (mm) values

From the values calculated in Step 3, we can see that the maximum value is M=392M = 392 and the minimum value is m=216m = -216.

Therefore, Mm=392(216)=392+216=608M - m = 392 - (-216) = 392 + 216 = 608.

Common Mistakes & Tips

  • Forgetting Endpoints: Always remember to evaluate the function at the endpoints of the interval.
  • Sign Errors: Be careful with negative signs when evaluating the function, especially when raising negative numbers to odd powers.
  • Factoring: Factoring the derivative simplifies finding the critical points and reduces the risk of algebraic errors.

Summary

To find the maximum and minimum values of the function f(x)=(x+3)2(x2)3f(x) = (x+3)^2(x-2)^3 on the interval [4,4][-4, 4], we first found the derivative f(x)f'(x), then determined the critical points by setting f(x)=0f'(x) = 0. We evaluated f(x)f(x) at the critical points and the endpoints of the interval. Finally, we identified the maximum and minimum values and calculated their difference, Mm=608M - m = 608.

Final Answer

The final answer is \boxed{608}, which corresponds to option (C).

Practice More Application of Derivatives Questions

View All Questions