Sign in
12thPass logo
New chatPYQ LibraryDoubtsRank report
Sign in to see Recents

Your guest activity stays on this device

Sign in to save progress →
Sign in

Differentiation question

2018 · 15 Apr · Shift 1 · Q46
Guest · filters and generic practice availableBrowsing as a guest · PYQ filters and generic practice are available. Sign in only for personalised features and saved progress.
  1. PYQ Library
  2. /JEE Main
  3. /Mathematics
  4. /Differentiation
  5. /2018 · 15 Apr · Shift 1 · Q46

Differentiation question

2018 · 15 Apr · Shift 1 · Q46

JEE MainMathematicsDifferentiationMCQ+4 / −1
If x2 + y2 + sin y = 4, then the value of d2ydx2{{{d^2}y} \over {d{x^2}}}dx2d2y​ at the point (−-− 2,0) is :
  1. A
    −-− 34
  2. B
    −-− 32
  3. C
    4
  4. D
    −-− 2
View written solutionFree

Correct answer: A

  1. Given equation

    x2+y2+ sin⁡y=4x^2 + y^2 + \,\sin y = 4x2+y2+siny=4

    We need to find:

    d2ydx2\frac{d^2 y}{dx^2}dx2d2y​

    at the point (−2,0)(-2,0)(−2,0).

  2. Differentiate implicitly once

    Differentiate both sides with respect to xxx:

    ddx(x2)+ddx(y2)+ddx(sin⁡y)=0\frac{d}{dx}(x^2) + \frac{d}{dx}(y^2) + \frac{d}{dx}(\sin y) = 0dxd​(x2)+dxd​(y2)+dxd​(siny)=0

    2x+2ydydx+cos⁡ydydx=02x + 2y\frac{dy}{dx} + \cos y\frac{dy}{dx} = 02x+2ydxdy​+cosydxdy​=0

    So,

    2x+(2y+cos⁡y)y′=02x + (2y + \cos y)y' = 02x+(2y+cosy)y′=0

    where y′=dydxy' = \frac{dy}{dx}y′=dxdy​.

    Hence,

    y′=−2x2y+cos⁡yy' = \frac{-2x}{2y + \cos y}y′=2y+cosy−2x​

  3. Find y′y'y′ at (−2,0)(-2,0)(−2,0)

    Substitute x=−2,y=0x=-2, y=0x=−2,y=0:

    y′=−2(−2)2(0)+cos⁡0=41=4y' = \frac{-2(-2)}{2(0) + \cos 0} = \frac{4}{1} = 4y′=2(0)+cos0−2(−2)​=14​=4

  4. Differentiate again

    From

    2x+(2y+cos⁡y)y′=02x + (2y + \cos y)y' = 02x+(2y+cosy)y′=0

    differentiate with respect to xxx:

    2+ddx[(2y+cos⁡y)y′]=02 + \frac{d}{dx}\left[(2y + \cos y)y'\right] = 02+dxd​[(2y+cosy)y′]=0

    Use product rule:

    2+(ddx(2y+cos⁡y))y′+(2y+cos⁡y)y′′=02 + \left(\frac{d}{dx}(2y + \cos y)\right)y' + (2y + \cos y)y'' = 02+(dxd​(2y+cosy))y′+(2y+cosy)y′′=0

    Now,

    ddx(2y+cos⁡y)=2y′−sin⁡y y′=(2−sin⁡y)y′\frac{d}{dx}(2y + \cos y) = 2y' - \sin y\, y' = (2 - \sin y)y'dxd​(2y+cosy)=2y′−sinyy′=(2−siny)y′

    Therefore,

    2+(2−sin⁡y)(y′)2+(2y+cos⁡y)y′′=02 + (2 - \sin y)(y')^2 + (2y + \cos y)y'' = 02+(2−siny)(y′)2+(2y+cosy)y′′=0

    So,

    y′′=−2−(2−sin⁡y)(y′)22y+cos⁡yy'' = \frac{-2 - (2 - \sin y)(y')^2}{2y + \cos y}y′′=2y+cosy−2−(2−siny)(y′)2​

  5. Evaluate at (−2,0)(-2,0)(−2,0)

    At y=0y=0y=0,

    sin⁡0=0,cos⁡0=1,y′=4\sin 0 = 0, \quad \cos 0 = 1, \quad y' = 4sin0=0,cos0=1,y′=4

    Hence,

    y′′=−2−(2−0)(4)22(0)+1y'' = \frac{-2 - (2-0)(4)^2}{2(0)+1}y′′=2(0)+1−2−(2−0)(4)2​

    y′′=−2−2⋅16y'' = -2 - 2\cdot 16y′′=−2−2⋅16

    y′′=−2−32=−34y'' = -2 - 32 = -34y′′=−2−32=−34

  6. Compare with options

    The correct option is:

    A: −34\boxed{\text{A: } -34}A: −34​

  7. Comparison with stored answer

    Stored correct answer is A, which matches our result.

PreviousNext

More from Differentiation

  • If f(x)=​cosx2sinxtanx​xx2x​12x1​​, then x→0lim​xf′(x)​2018 · MCQ
  • If f(x) = sin-1 (1+9x2×3x​), then f'(−21​) equals :2018 · MCQ
  • If x=2cosec−1​ and y=2sec−1t​(∣t∣≥1), then dxdy​ is equal to :2018 · MCQ
  • If y = [x+x2−1​]15+[x−x2−1​]15, then (x2 − 1) dx2d2y​+xdxdy​ is equal to :2017 · MCQ
  • Let f be a polynomial function such that f (3x) = f ' (x) . f '' (x), for all x ∈ R. Then :2017 · MCQ
  • If for x∈(0,41​), the derivatives of tan−1(1−9x36xx​​) is x​.g(x), then g(x) equals2017 · MCQ
  • If g is the inverse of a function f and f′(x)=1+x51​, then g′(x) is equal to:2014 · MCQ
  • If y=sec(tan−1x), then dxdy​ at x=1 is equal to :2013 · MCQ