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Application of Derivatives question

2021 · 26 Feb · Shift 1 · Q34
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  5. /2021 · 26 Feb · Shift 1 · Q34

Application of Derivatives question

2021 · 26 Feb · Shift 1 · Q34

JEE MainMathematicsApplication of DerivativesMCQ+4 / −1
The maximum slope of the curve y=12x4−5x3+18x2−19xy = {1 \over 2}{x^4} - 5{x^3} + 18{x^2} - 19xy=21​x4−5x3+18x2−19x occurs at the point :
  1. A
    (3,212)\left( {3,{{21} \over 2}} \right)(3,221​)
  2. B
    (0, 0)
  3. C
    (2, 9)
  4. D
    (2, 2)
View written solutionFree

Correct answer: D

  1. The slope of the curve is given by the derivative:

y=12x4−5x3+18x2−19xy=\frac{1}{2}x^4-5x^3+18x^2-19xy=21​x4−5x3+18x2−19x

So,

dydx=2x3−15x2+36x−19\frac{dy}{dx}=2x^3-15x^2+36x-19dxdy​=2x3−15x2+36x−19

  1. We need the maximum slope, i.e. the maximum value of

m(x)=dydx=2x3−15x2+36x−19m(x)=\frac{dy}{dx}=2x^3-15x^2+36x-19m(x)=dxdy​=2x3−15x2+36x−19

To find where this slope is extremum, differentiate again:

m′(x)=d2ydx2=6x2−30x+36m'(x)=\frac{d^2y}{dx^2}=6x^2-30x+36m′(x)=dx2d2y​=6x2−30x+36

=6(x2−5x+6)=6(x−2)(x−3)=6(x^2-5x+6)=6(x-2)(x-3)=6(x2−5x+6)=6(x−2)(x−3)

Thus critical points for the slope occur at

x=2,  3x=2,\;3x=2,3

  1. To determine where the slope is maximum, use the third derivative:

d3ydx3=12x−30\frac{d^3y}{dx^3}=12x-30dx3d3y​=12x−30

  • At x=2x=2x=2: d3ydx3=24−30=−6<0\frac{d^3y}{dx^3}=24-30=-6<0dx3d3y​=24−30=−6<0 So dydx\dfrac{dy}{dx}dxdy​ has a local maximum at x=2x=2x=2.

  • At x=3x=3x=3: d3ydx3=36−30=6>0\frac{d^3y}{dx^3}=36-30=6>0dx3d3y​=36−30=6>0 So dydx\dfrac{dy}{dx}dxdy​ has a local minimum at x=3x=3x=3.

Hence the maximum slope occurs at x=2x=2x=2.

  1. Now find the corresponding point on the curve:

y(2)=12(24)−5(23)+18(22)−19(2)y(2)=\frac{1}{2}(2^4)-5(2^3)+18(2^2)-19(2)y(2)=21​(24)−5(23)+18(22)−19(2)

=12(16)−5(8)+18(4)−38=\frac{1}{2}(16)-5(8)+18(4)-38=21​(16)−5(8)+18(4)−38

=8−40+72−38=2=8-40+72-38=2=8−40+72−38=2

So the point is

(2,2)(2,2)(2,2)

  1. Checking options:
  • A: (3,212)\left(3,\frac{21}{2}\right)(3,221​) — no
  • B: (0,0)(0,0)(0,0) — no
  • C: (2,9)(2,9)(2,9) — no
  • D: (2,2)(2,2)(2,2) — yes

Therefore, the maximum slope occurs at

(2,2)\boxed{(2,2)}(2,2)​

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