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Differentiation question

2023 · 6 Apr · Shift 1 · Q35
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  5. /2023 · 6 Apr · Shift 1 · Q35

Differentiation question

2023 · 6 Apr · Shift 1 · Q35

JEE MainMathematicsDifferentiationMCQ+4 / −1
If 2xy+3yx=202 x^{y}+3 y^{x}=202xy+3yx=20, then dydx\frac{d y}{d x}dxdy​ at (2,2)(2,2)(2,2) is equal to :
  1. A
    −(3+log⁡e164+log⁡e8)-\left(\frac{3+\log _{e} 16}{4+\log _{e} 8}\right)−(4+loge​83+loge​16​)
  2. B
    −(2+log⁡e83+log⁡e4)-\left(\frac{2+\log _{e} 8}{3+\log _{e} 4}\right)−(3+loge​42+loge​8​)
  3. C
    −(3+log⁡e82+log⁡e4)-\left(\frac{3+\log _{e} 8}{2+\log _{e} 4}\right)−(2+loge​43+loge​8​)
  4. D
    −(3+log⁡e42+log⁡e8)-\left(\frac{3+\log _{e} 4}{2+\log _{e} 8}\right)−(2+loge​83+loge​4​)
View written solutionFree

Correct answer: B

  1. Given equation

    2xy+3yx=202x^y+3y^x=202xy+3yx=20

    We need to find dydx\dfrac{dy}{dx}dxdy​ at the point (2,2)(2,2)(2,2).

  2. Differentiate implicitly

    Recall:

    • For xyx^yxy, taking log helps: xy=eyln⁡xx^y=e^{y\ln x}xy=eylnx So, ddx(xy)=xy(dydxln⁡x+yx)\frac{d}{dx}(x^y)=x^y\left(\frac{dy}{dx}\ln x+\frac{y}{x}\right)dxd​(xy)=xy(dxdy​lnx+xy​)

    • For yxy^xyx, yx=exln⁡yy^x=e^{x\ln y}yx=exlny So, ddx(yx)=yx(ln⁡y+xydydx)\frac{d}{dx}(y^x)=y^x\left(\ln y+\frac{x}{y}\frac{dy}{dx}\right)dxd​(yx)=yx(lny+yx​dxdy​)

    Differentiating 2xy+3yx=202x^y+3y^x=202xy+3yx=20 with respect to xxx:

    2 xy(dydxln⁡x+yx)+3 yx(ln⁡y+xydydx)=02\,x^y\left(\frac{dy}{dx}\ln x+\frac{y}{x}\right)+3\,y^x\left(\ln y+\frac{x}{y}\frac{dy}{dx}\right)=02xy(dxdy​lnx+xy​)+3yx(lny+yx​dxdy​)=0

  3. Substitute the point (2,2)(2,2)(2,2)

    At (x,y)=(2,2)(x,y)=(2,2)(x,y)=(2,2):

    xy=22=4,yx=22=4,ln⁡x=ln⁡2,ln⁡y=ln⁡2x^y=2^2=4, \quad y^x=2^2=4, \quad \ln x=\ln 2, \quad \ln y=\ln 2xy=22=4,yx=22=4,lnx=ln2,lny=ln2

    Hence,

    2⋅4(dydxln⁡2+22)+3⋅4(ln⁡2+22dydx)=02\cdot 4\left(\frac{dy}{dx}\ln 2+\frac{2}{2}\right)+3\cdot 4\left(\ln 2+\frac{2}{2}\frac{dy}{dx}\right)=02⋅4(dxdy​ln2+22​)+3⋅4(ln2+22​dxdy​)=0

    8(dydxln⁡2+1)+12(ln⁡2+dydx)=08\left(\frac{dy}{dx}\ln 2+1\right)+12\left(\ln 2+\frac{dy}{dx}\right)=08(dxdy​ln2+1)+12(ln2+dxdy​)=0

  4. Simplify

    8dydxln⁡2+8+12ln⁡2+12dydx=08\frac{dy}{dx}\ln 2+8+12\ln 2+12\frac{dy}{dx}=08dxdy​ln2+8+12ln2+12dxdy​=0

    Grouping terms in dydx\dfrac{dy}{dx}dxdy​:

    dydx(8ln⁡2+12)+(8+12ln⁡2)=0\frac{dy}{dx}(8\ln 2+12)+(8+12\ln 2)=0dxdy​(8ln2+12)+(8+12ln2)=0

    Therefore,

    dydx=−8+12ln⁡212+8ln⁡2\frac{dy}{dx}=-\frac{8+12\ln 2}{12+8\ln 2}dxdy​=−12+8ln28+12ln2​

    Divide numerator and denominator by 444:

    dydx=−2+3ln⁡23+2ln⁡2\frac{dy}{dx}=-\frac{2+3\ln 2}{3+2\ln 2}dxdy​=−3+2ln22+3ln2​

  5. Convert into option form

    Using: ln⁡8=3ln⁡2,ln⁡4=2ln⁡2\ln 8=3\ln 2, \qquad \ln 4=2\ln 2ln8=3ln2,ln4=2ln2

    we get

    dydx=−(2+ln⁡83+ln⁡4)\frac{dy}{dx}=-\left(\frac{2+\ln 8}{3+\ln 4}\right)dxdy​=−(3+ln42+ln8​)

  6. Match with options

    This is exactly Option B.


Final Answer:

−(2+log⁡e83+log⁡e4)\boxed{-\left(\frac{2+\log_e 8}{3+\log_e 4}\right)}−(3+loge​42+loge​8​)​

So the correct option is B.

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