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Application of Derivatives question

2008 · Shift 1 · Q45
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  5. /2008 · Shift 1 · Q45

Application of Derivatives question

2008 · Shift 1 · Q45

JEE AdvancedMathematicsApplication of DerivativesMCQ+3 / −1
The total number of local maxima and local minima of the function f(x)={(2+x)3,−3<x≤−1x2/3,−1<x<2f(x) = \left\{ {\begin{matrix} {{{(2 + x)}^3},} & { - 3 \lt x \le - 1} \\ {{x^{2/3}},} & { - 1 \lt x \lt 2} \\ \end{matrix} } \right.f(x)={(2+x)3,x2/3,​−3<x≤−1−1<x<2​ is
  1. A
    0
  2. B
    1
  3. C
    2
  4. D
    3
View written solutionFree

Correct answer: C

The user wants me to find the total number of local maxima and local minima for the given piecewise function.

Function: f(x)={(2+x)3,−3<x≤−1x2/3,−1<x<2f(x) = \left\{ \begin{matrix} {{{(2 + x)}^3},} & { - 3 < x \le - 1} \\ {{x^{2/3}},} & { - 1 < x < 2} \\ \end{matrix} \right.f(x)={(2+x)3,x2/3,​−3<x≤−1−1<x<2​

To find the local maxima and minima, we need to find the critical points of the function. Critical points occur where the derivative f'(x) is zero or undefined.

Step 1: Analyze the first piece of the function, f(x)=(2+x)3f(x) = (2 + x)^3f(x)=(2+x)3 for -3 < x <= -1

  1. Find the derivative: f′(x)=d/dx[(2+x)3]=3(2+x)2f'(x) = d/dx [(2 + x)^3] = 3(2 + x)^2f′(x)=d/dx[(2+x)3]=3(2+x)2

  2. Find critical points by setting f'(x) = 0: 3(2+x)2=03(2 + x)^2 = 03(2+x)2=0 2 + x = 0 x = -2

  3. Check if x = -2 is in the interval (-3, -1]: Yes, -3 < -2 <= -1.

  4. Use the First Derivative Test to classify x = -2: We examine the sign of f′(x)=3(2+x)2f'(x) = 3(2 + x)^2f′(x)=3(2+x)2 around x = -2.

    • For x < -2 (e.g., x = -2.5), f′(x)=3(2−2.5)2=3(−0.5)2>0f'(x) = 3(2 - 2.5)^2 = 3(-0.5)^2 > 0f′(x)=3(2−2.5)2=3(−0.5)2>0.
    • For x > -2 (e.g., x = -1.5), f′(x)=3(2−1.5)2=3(0.5)2>0f'(x) = 3(2 - 1.5)^2 = 3(0.5)^2 > 0f′(x)=3(2−1.5)2=3(0.5)2>0. Since f'(x) does not change sign at x = -2 (it is positive on both sides), x = -2 is a point of inflection, not a local maximum or minimum.

Step 2: Analyze the second piece of the function, f(x)=x(2/3)f(x) = x^(2/3)f(x)=x(2/3) for -1 < x < 2

  1. Find the derivative: f′(x)=d/dx[x(2/3)]=(2/3)x(−1/3)=2/(3∗x(1/3))f'(x) = d/dx [x^(2/3)] = (2/3)x^(-1/3) = 2 / (3 * x^(1/3))f′(x)=d/dx[x(2/3)]=(2/3)x(−1/3)=2/(3∗x(1/3))

  2. Find critical points:

    • f'(x) = 0: 2/(3∗x(1/3))=02 / (3 * x^(1/3)) = 02/(3∗x(1/3))=0 has no solution.
    • f'(x) is undefined when the denominator is zero, which occurs at x = 0.
  3. Check if x = 0 is in the interval (-1, 2): Yes, -1 < 0 < 2.

  4. Use the First Derivative Test to classify x = 0: We examine the sign of f′(x)=2/(3∗x(1/3))f'(x) = 2 / (3 * x^(1/3))f′(x)=2/(3∗x(1/3)) around x = 0.

    • For x < 0 (e.g., x = -0.1), x(1/3)x^(1/3)x(1/3) is negative, so f'(x) < 0. The function is decreasing.
    • For x > 0 (e.g., x = 0.1), x(1/3)x^(1/3)x(1/3) is positive, so f'(x) > 0. The function is increasing. Since f'(x) changes from negative to positive at x = 0, the function has a local minimum at x = 0.

Step 3: Analyze the point where the function definition changes, x = -1

Points where the function definition changes can also be local extrema if the function is not differentiable there. We need to check the behavior of the function around x = -1.

  1. Check for continuity at x = -1:

    • Value at x = -1: f(−1)=(2+(−1))3=13=1f(-1) = (2 + (-1))^3 = 1^3 = 1f(−1)=(2+(−1))3=13=1.
    • Limit from the right: limx−>−1+f(x)=limx−>−1+x(2/3)=(−1)(2/3)=((−1)2)(1/3)=1(1/3)=1lim_{x->-1^+} f(x) = lim_{x->-1^+} x^(2/3) = (-1)^(2/3) = ((-1)^2)^(1/3) = 1^(1/3) = 1limx−>−1+​f(x)=limx−>−1+​x(2/3)=(−1)(2/3)=((−1)2)(1/3)=1(1/3)=1. Since the value of the function and the limit from the right are equal, the function is continuous at x = -1.
  2. Check differentiability at x = -1 (or use the first derivative test across the point):

    • For x in (-3, -1), we found f′(x)=3(2+x)2f'(x) = 3(2 + x)^2f′(x)=3(2+x)2. As x approaches -1 from the left, f'(x) approaches 3(2−1)2=33(2 - 1)^2 = 33(2−1)2=3, which is positive. So, f(x) is increasing to the left of x = -1.
    • For x in (-1, 0), we found f′(x)=2/(3∗x(1/3))f'(x) = 2 / (3 * x^(1/3))f′(x)=2/(3∗x(1/3)). For x in this interval, x(1/3)x^(1/3)x(1/3) is negative, so f'(x) is negative. So, f(x) is decreasing to the right of x = -1.

    Since the function is increasing to the left of x = -1 and decreasing to the right, x = -1 is a point of local maximum.

Step 4: Conclusion

We have found the following:

  • x = -2 is a point of inflection.
  • x = -1 is a local maximum.
  • x = 0 is a local minimum.

The total number of local maxima and local minima is one maximum plus one minimum, which is 2.

This corresponds to option C.

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