Questions   
Sort: 
 #1
avatar+26367 
+3

Let F(x) be the real-valued function defined for all real x except for x = 0 and x = 1 and satisfying

the functional equation

\(F(x) + F\left(\frac{x-1}x\right) = 1+x.\)

F(x) + F\left(\frac{x-1}x\right) = 1+x.

Find the F(x) satisfying these conditions.

Write F(x) as a rational function with expanded polynomials in the numerator and denominator.

 

\(\begin{array}{|lrclcl|} \hline & F(x) + F\left(\frac{x-1}x\right) &=& 1+x \qquad (1) \\\\ \text{Set in (1) }x=\frac{x-1}{x}: & F\left(\frac{x-1}x\right) + F\left(\frac1{1-x}\right) &=& 1+\frac{x-1}{x} \qquad (2) \\\\ \text{Set in (1) }x=\frac1{1-x}: & F\left(\frac1{1-x}\right) + F(x) &=& 1+\frac1{1-x} \qquad (3) \\\\ \hline \\ (1) - (2) + (3): & F(x) + F\left(\frac{x-1}x\right) \\ & - \Big(F\left(\frac{x-1}x\right) + F\left(\frac1{1-x}\right) \Big) \\ & + F\left(\frac1{1-x}\right) + F(x) &=& 1+x -(1+\frac{x-1}{x}) + 1+\frac1{1-x} \\\\ & F(x) + F\left(\frac{x-1}x\right) \\ & -F\left(\frac{x-1}x\right) - F\left(\frac1{1-x}\right) \\ & + F\left(\frac1{1-x}\right) + F(x) &=& 1+x -(1+\frac{x-1}{x}) + 1+\frac1{1-x} \\\\ & 2F(x) &=& 1+x -(1+\frac{x-1}{x}) + 1+\frac1{1-x} \\ & 2F(x) &=& 1+x -1-\frac{x-1}{x} + 1+\frac1{1-x} \\ & 2F(x) &=& 1+x -\frac{x-1}{x} +\frac1{1-x} \\ & 2F(x) &=& 1+x +\frac{1-x}{x} +\frac1{1-x} \\\\ & 2F(x) &=& \dfrac{(1+x)x(1-x)+(1-x)^2+x}{x(1-x)} \\\\ & 2F(x) &=& \dfrac{(1-x^2)x+1-2x+x^2+x}{x(1-x)} \\\\ & 2F(x) &=& \dfrac{(1-x^2)x+1-x+x^2}{x(1-x)} \\\\ & 2F(x) &=& \dfrac{ x-x^3 +1-x+x^2}{x(1-x)} \\\\ & 2F(x) &=& \dfrac{ 1+x^2-x^3}{x(1-x)} \\\\ & F(x) &=& \dfrac{ 1+x^2-x^3}{2x(1-x)} \\\\ & \mathbf{F(x)} & \mathbf{=} & \mathbf{\dfrac{ 1+x^2-x^3}{2x - 2x^2}} \\ \hline \end{array}\)

 

graph:

 

laugh

May 23, 2018
 #4
avatar+26367 
+1

Use logarithmic differentation to find the derivative of the function.

\(y=\sqrt{x}{e}^{({x}^{2})}({{x}^{2}+1})^{10}\)

y=\sqrt{x}{e}^{(x^2)}(x^2+1)^{10}

Anyone who knows how to solve for the answer and can write down the steps?

 

Formula

\(\begin{array}{|rclcl|} \hline \left(~\ln f(x)~ \right)' = \dfrac{f'(x)}{f(x)} \\ \hline \end{array} \)

 

1. logarithm of both sides

\(\begin{array}{|rcll|} \hline y &=& \sqrt{x}{e}^{(x^2)}(x^2+1)^{10} \quad & | \quad \text{$\ln()$ both sides} \\ \ln(y) &=& \ln(\sqrt{x})+\ln({e}^{(x^2)}) + \ln( (x^2+1)^{10} ) \\ \ln(y) &=& \ln(x^{\frac12})+\ln({e}^{(x^2)}) + \ln( (x^2+1)^{10} ) \quad & | \quad \text{Formula: $\ln(a^b) = b\ln(a) $} \\ \ln(y) &=& \frac12\ln(x)+x^2\ln(e) + 10\ln(x^2+1) \quad & | \quad \text{Formula: $\ln(e) = 1 $} \\ \ln(y) &=& \frac12\ln(x)+x^2 + 10\ln(x^2+1) \\ \hline \end{array}\)

 

2. derivation of both sides

\(\begin{array}{|rcll|} \hline \ln(y) &=& \frac12\ln(x)+x^2 + 10\ln(x^2+1) \quad & | \quad \text{derivate both sides} \\ \Big(\ln(y)\Big)' &=& \left(\frac12\ln(x) \right)' + \left(x^2 \right)' + \left(10\ln(x^2+1) \right)' \\\\ \dfrac{y'}{y} &=& \frac12\cdot \frac1x + 2x + 10\cdot \frac{2x}{x^2+1} \\\\ \dfrac{y'}{y} &=& \frac{1}{2x} + 2x + \frac{20x}{x^2+1} \quad & | \quad \cdot y \\\\ y' &=& y\cdot \left( \frac{1}{2x} + 2x + \frac{20x}{x^2+1} \right) \quad & | \quad y = \sqrt{x}e^{(x^2)}(x^2+1)^{10} \\\\ y' &=& \sqrt{x}e^{(x^2)}(x^2+1)^{10} \cdot \left( \frac{1}{2x} + 2x + \frac{20x}{x^2+1} \right) \\\\ y' &=& x^{\frac12}e^{(x^2)}(x^2+1)^{10} \cdot \left( \frac{1}{2x} + 2x + \frac{20x}{x^2+1} \right) \\\\ y' &=& \frac{ x^{\frac12}{e}^{(x^2)}(x^2+1)^{10} }{2x} + x^{\frac12}e^{(x^2)}(x^2+1)^{10} \cdot 2x \\ &+& x^{\frac12}e^{(x^2)}(x^2+1)^{10} \cdot \frac{20x}{x^2+1} \\\\ y' &=& \frac{ e^{(x^2)}(x^2+1)^{10} }{2x^{1-\frac12}} + 2{e}^{(x^2)}x^{\frac12+1}(x^2+1)^{10}\\ &+& 20e^{(x^2)} x^{\frac12+1}(x^2+1)^{10} \cdot \frac{1}{(x^2+1)^1} \\\\ y' &=& \frac{ e^{(x^2)}(x^2+1)^{10} }{2x^{\frac12}} + 2{e}^{(x^2)}x^{\frac32}(x^2+1)^{10}\\ &+& 20e^{(x^2)} x^{\frac32}(x^2+1)^{10-1} \\\\ y' &=& \frac{ e^{(x^2)}(x^2+1)^{10} }{2\sqrt{x}} + 2{e}^{(x^2)}x^{\frac32}(x^2+1)^{10}\\ &+& 20e^{(x^2)} x^{\frac32}(x^2+1)^{9} \\ \hline \end{array} \)

 

\(\begin{array}{rcll} \mathbf{ y' } & \mathbf{=} & \mathbf{ \dfrac{ e^{(x^2)}(x^2+1)^{10} }{2\sqrt{x}} + 2{e}^{(x^2)}x^{\frac32}(x^2+1)^{10} + 20e^{(x^2)} x^{\frac32}(x^2+1)^{9} } \\ \end{array} \)

 

laugh

May 23, 2018
 #4
avatar
0
May 23, 2018
May 22, 2018
 #1
avatar+9466 
+2

The circle travels outward at a speed of 60 cm/s, which means the radius of the circle is increasing at a rate of 60 cm per second.

 

radius after  0  sec  =  0

radius after  1  sec  =  60

radius after  2  sec  =  60 + 60  =  60(2)

radius after  3  sec  =  60 + 60 + 60  =  60(3)

radius after  s  sec  =  60s

 

Or we can say...

 

radius  =  60s    , where   s  is the number of seconds after the stone hit the water.

 

Let   s  =  the number of seconds after the stone hit the water   and   a  =  the area of the circle

 

We know that the equation for the area of a circle is...

 

a  =  pi (radius)2

                            Substitute  60s  in for radius.

a  =  pi( 60s )2

 

a  =  3600 pi s2

 

This equation tells us the area, a , at any  s . But we want to know the rate of change in  a  per change in  s  at any  s . So take  d / ds  of both sides of the equation.

 

da / ds  =  d / ds [ 3600 pi s2 ]

 

da / ds  =  ( 3600 )( pi )( d/ds s2 )

 

da / ds  =  ( 3600 )( pi )( 2s )

 

da / ds  =  7200 pi s

 

To find the rate at which the area is increasing after 1 second, plug in  1 for  s  and solve for  da / ds .

 

(a)   when  s  =  1 ,          da / ds   =   7200 pi (1)   =   7200 pi     (sq cm per second)

 

(b)   when  s  =  3 ,          da / ds   =   7200 pi (2)   =   14400 pi

 

I'll let you do part  (c)  .

 

Notice that the bigger the number of seconds, the bigger the rate at which the area is increasing.

May 22, 2018

7 Online Users

avatar
avatar
avatar
avatar
avatar