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Differentiation question

2009 · Shift 0 · Q31
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  5. /2009 · Shift 0 · Q31

Differentiation question

2009 · Shift 0 · Q31

JEE MainMathematicsDifferentiationMCQ+4 / −1
Let yyy be an implicit function of xxx defined by x2x−2xxcot⁡ y−1=0{x^{2x}} - 2{x^x}\cot \,y - 1 = 0x2x−2xxcoty−1=0. Then y′(1)y'(1)y′(1) equals
  1. A
    111
  2. B
    log⁡ 2\log \,2log2
  3. C
    −log⁡ 2-\log \,2−log2
  4. D
    −1-1−1
View written solutionFree

Correct answer: D

  1. Given equation

    x2x−2xxcoty−1=0x^{2x}-2x^x\\cot y-1=0x2x−2xxcoty−1=0

    We need to find y′(1)y'(1)y′(1).

  2. First find yyy at x=1x=1x=1

    Substitute x=1x=1x=1:

    12−2⋅1⋅cot⁡y−1=01^{2}-2\cdot 1\cdot \cot y-1=012−2⋅1⋅coty−1=0 1−2cot⁡y−1=01-2\cot y-1=01−2coty−1=0 −2cot⁡y=0-2\cot y=0−2coty=0 cot⁡y=0\cot y=0coty=0

    Hence,

    y=π2+nπy=\frac{\pi}{2}+n\piy=2π​+nπ

    Therefore,

    csc⁡2y=1\csc^2 y=1csc2y=1

  3. Differentiate implicitly

    The equation is

    x2x−2xxcot⁡y−1=0x^{2x}-2x^x\cot y-1=0x2x−2xxcoty−1=0

    Differentiate both sides w.r.t. xxx.

    Recall:

    ddx(xx)=xx(ln⁡x+1)\frac{d}{dx}(x^x)=x^x(\ln x+1)dxd​(xx)=xx(lnx+1)

    Also,

    x2x=e2xln⁡x  ⟹  ddx(x2x)=x2x(2ln⁡x+2)x^{2x}=e^{2x\ln x} \implies \frac{d}{dx}(x^{2x})=x^{2x}(2\ln x+2)x2x=e2xlnx⟹dxd​(x2x)=x2x(2lnx+2)

    Now differentiate term by term:

    ddx(x2x)−2ddx(xxcot⁡y)=0\frac{d}{dx}(x^{2x})-2\frac{d}{dx}(x^x\cot y)=0dxd​(x2x)−2dxd​(xxcoty)=0

    Using product rule on xxcot⁡yx^x\cot yxxcoty:

    ddx(xxcot⁡y)=xx(ln⁡x+1)cot⁡y+xxddx(cot⁡y)\frac{d}{dx}(x^x\cot y)=x^x(\ln x+1)\cot y+x^x\frac{d}{dx}(\cot y)dxd​(xxcoty)=xx(lnx+1)coty+xxdxd​(coty)

    and

    ddx(cot⁡y)=−csc⁡2y y′\frac{d}{dx}(\cot y)=-\csc^2 y\, y'dxd​(coty)=−csc2yy′

    So,

    x2x(2ln⁡x+2)−2[xx(ln⁡x+1)cot⁡y−xxcsc⁡2y y′]=0x^{2x}(2\ln x+2)-2\left[x^x(\ln x+1)\cot y-x^x\csc^2 y\, y'\right]=0x2x(2lnx+2)−2[xx(lnx+1)coty−xxcsc2yy′]=0

  4. Substitute x=1x=1x=1

    At x=1x=1x=1,

    x2x=1,xx=1,ln⁡1=0,cot⁡y=0,csc⁡2y=1x^{2x}=1, \quad x^x=1, \quad \ln 1=0, \quad \cot y=0, \quad \csc^2 y=1x2x=1,xx=1,ln1=0,coty=0,csc2y=1

    Therefore,

    1⋅(2)−2[1⋅(1)⋅0−1⋅1⋅y′]=01\cdot (2)-2\left[1\cdot (1)\cdot 0-1\cdot 1\cdot y'\right]=01⋅(2)−2[1⋅(1)⋅0−1⋅1⋅y′]=0

    2−2(−y′)=02-2( - y')=02−2(−y′)=0

    2+2y′=02+2y'=02+2y′=0

    y′=−1y'=-1y′=−1

  5. Final answer

    y′(1)=−1y'(1)=-1y′(1)=−1

    So the correct option is D.

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