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Calculus Level 4

Let y y be an implicit function of x x defined by x 2 x 2 x x cot y 1 = 0 x^{2x}-2x^x\cot{y}-1=0 .

Find the value of y ( 1 ) y' (1) , where y y' denotes the first derivative of y y .

ln 2 \ln 2 ln 2 -\ln 2 1 -1 None of these choices 1 1

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2 solutions

Chew-Seong Cheong
May 25, 2016

x 2 x 2 x x cot y 1 = 0 When x = 1 1 2 cot ( y ( 1 ) ) 1 = 0 cot ( y ( 1 ) ) = 0 y ( 1 ) = π 2 \begin{aligned} x^{2x}-2x^x\cot y -1& =0 \quad \quad \small \color{#3D99F6}{\text{When }x=1} \\ 1 - 2 \cot (y(1)) - 1 & = 0 \\ \implies \cot (y(1)) & = 0 \\ y(1) & = \frac{\pi}{2} \end{aligned}

Now, we have:

x 2 x 2 x x cot y 1 = 0 Differentiate both sides with respect to x 2 ( ln x + 1 ) x 2 x 2 ( ln x + 1 ) cot y + 2 x x csc 2 y d y d x = 0 When x = 1 2 0 + 2 csc 2 π 2 y ( 1 ) = 0 y ( 1 ) = 1 \begin{aligned} x^{2x}-2x^x\cot y -1& =0 \quad \quad \small \color{#3D99F6}{\text{Differentiate both sides with respect to }x} \\ 2(\ln x + 1)x^{2x} - 2(\ln x + 1)\cot y + 2x^x \csc^2 y \frac{dy}{dx} & = 0 \quad \quad \small \color{#3D99F6}{\text{When }x=1} \\ 2 - 0 + 2 \csc^2 \frac{\pi}{2} y'(1) & = 0 \\ \quad y'(1) & = \boxed{-1} \end{aligned}

However y is not a function of x. There is simply an implicit relation between x and y. Coincidently whether you take pi/2 or minus pi/2, the derivative is the same.

NILAY PANDE - 3 years, 6 months ago
Sabhrant Sachan
May 25, 2016

We are Given x 2 x 2 x x cot y 1 = 0 ( x x ) 2 2 ( x x ) ( cot y ) + cot 2 y 1 cot 2 y = 0 ( x x cot y ) 2 = 1 + cot 2 y x x cot y = csc y x x = cot y + csc y Taking Derivative w.r.t x both side x x ( 1 + ln x ) = csc 2 y × y csc y cot y × y x x ( 1 + ln x ) = csc y ( csc y + cot y ) y y = x x ( 1 + ln x ) x x csc y y ( 1 ) = 1 1 ( 1 + ln 1 ) csc ( y ( 1 ) ) sin ( y ( 1 ) ) From our Original Equation , 1 2 2 × 1 1 cot y ( 1 ) 1 = 0 y ( 1 ) = π 2 y ( 1 ) = sin ( y ( 1 ) ) 1 \text{ We are Given } x^{2x}-2x^x\cot{y}-1=0 \\ (x^x)^2-2(x^x)(\cot{y})+\cot^2{y}-1-\cot^2{y}=0 \\ (x^x-\cot{y})^2=1+\cot^2{y} \\ x^x-\cot{y}=\csc{y} \\ x^x=\cot{y}+\csc{y} \\ \text{Taking Derivative w.r.t x both side} \\ x^x(1+\ln{x})=-\csc^2{y}\times y^{'}-\csc{y}\cot{y}\times y^{'} \\ x^x(1+\ln{x})=-\csc{y}(\csc{y}+\cot{y})y^{'} \\ y^{'}=-\dfrac{x^x(1+\ln{x})}{x^x\csc{y}} \\ y^{'}(1)=-\dfrac{1^1(1+\ln{1})}{\csc{(y(1))}} \implies -\sin{(y(1))} \\ \text{From our Original Equation , } 1^{2}-2 \times 1^1\cot{y(1)}-1=0 \implies y(1)=\dfrac{\pi}{2} \\ y^{'}(1)=-\sin{(y(1))} \implies \color{#3D99F6}{\boxed{-1}}

@Sabhrant Sachan What does mean w.r.t.x in your solution?????????????????????????????????????????????????????????????????????? \text { What does mean w.r.t.x in your solution?????????????????????????????????????????????????????????????????????? }

. . - 2 months, 3 weeks ago

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"with respect to x"

Sabhrant Sachan - 1 month, 3 weeks ago

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