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Calculus

GATE CSE 2023 · Q28

MSQ1 markEasy

Time

Let f(x)=x3+15x2−33x−36f(x) = x^3 + 15x^2 - 33x - 36 be a real-valued function. Which of the following statements is/are TRUE?

Select all that applyOne or more correct

Pick every correct option

Solution

Step-by-step verified solution

Official answer: B) f(x) has a local maximum.; D) f(x) has a local minimum.

Calculus

To determine the local extrema of the function f(x)=x3+15x2−33x−36f(x) = x^3 + 15x^2 - 33x - 36, find its first derivative:

f′(x)=3x2+30x−33f^{\prime}(x) = 3x^2 + 30x - 33

Set f′(x)=0f^{\prime}(x) = 0 to find the critical points:

3(x2+10x−11)=03(x^2 + 10x - 11) = 0
3(x+11)(x−1)=03(x + 11)(x - 1) = 0

Thus, the critical points are x=−11x = -11 and x=1x = 1.

Now, compute the second derivative:

f′′(x)=6x+30f^{\prime\prime}(x) = 6x + 30

Evaluate the second derivative at the critical points:

  1. At x=−11x = -11:

    f′′(−11)=6(−11)+30=−36<0f^{\prime\prime}(-11) = 6(-11) + 30 = -36 < 0

    Since $f''(-11) < 0$, $f(x)$ has alocal maximum at x=−11x = -11.

  2. At x=1x = 1:

    f′′(1)=6(1)+30=36>0f^{\prime\prime}(1) = 6(1) + 30 = 36 > 0

    Since $f''(1) > 0$, $f(x)$ has alocal minimum at x=1x = 1.

Therefore, statements (B) and (D) are TRUE, while (A) and (C) are FALSE.

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This Calculus previous year question is Q28 of the GATE CSE 2023 paper. Find more GATE CSE PYQs on Calculus in Engineering Mathematics, or browse all GATE CSE PYQs.

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GateAI solution cardGATE CSE 2023 · Q28 · MSQ

Let f(x) = x3 + 15x2 - 33x - 36 be a real-valued function. Which of the following statements is/are TRUE?

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