Differential Equation

Calculus Level 2

Let y ( x ) y(x) be a function satisfying d y d x = 8 y 2 y 2 \dfrac{dy}{dx}=8y-2y^2 and y ( 2016 ) = 1 y(2016)=1 . Compute lim x y ( x ) \displaystyle \lim_{x \rightarrow \infty}y(x) .


The answer is 4.

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2 solutions

Althought this differential equation is a particular case of Bernouilli differential equation I'm going to use the variable separated method what is not absolutely formal but it works. d y d x = 8 y 2 y 2 , y ( 2016 ) = 1 \dfrac{dy}{dx} = 8y - 2y^2,\quad y(2016) = 1 d y 8 y 2 y 2 = d x \dfrac{dy}{8y - 2y^2} = dx \Rightarrow 1 8 y 2 y 2 d y = d x \int \frac{1}{8y -2y^2} dy = \int dx \Rightarrow 1 4 2 y d y + 1 8 4 y d y = x + C (C constant) \int \frac{\frac{1}{4}}{2y} dy + \int \frac{\frac{1}{8}}{4 -y} dy = x + C \space \text{ (C constant)} \Rightarrow 1 8 ( ln y ln 4 y ) = x + C \frac{1}{8} \cdot (\ln |y| - \ln |4 - y|) = x + C \Rightarrow e 1 8 ln y 4 y = K e x (K constant) \large {e^{\frac{1}{8} \cdot \ln \frac{|y|}{|4 - y|}}} = Ke^x \text{ (K constant)} \Rightarrow I'm going to consider A = y 4 y > 0 A = \frac{y}{4 - y} > 0 , if it was A < 0 A < 0 we can reason in the same way and we would get the same result. it depends on y(2016) = 1 ( y 4 y ) 1 8 = K e x ( y ( 2016 ) = 1 K 0 ) \left(\frac {y}{4 - y}\right)^{\frac{1}{8}} = Ke^x \quad (y(2016) = 1 \Rightarrow K \neq 0) \Rightarrow Powering to 8 and rearranging y ( 1 + k e 8 x ) = 4 k e 8 x (k constant) k 0 y \cdot (1 + ke^{8x}) = 4ke^{8x} \quad \text{(k constant)} \quad k \neq 0 \Rightarrow y = 4 k e 8 x 1 + k e 8 x lim x y = 4. \displaystyle y = \frac{4ke^{8x}}{1 + ke^{8x}} \Rightarrow \lim_{x \to \infty} y = 4. If we consider this equation like a logistic differential equation about the population growth, it could be interpreted saying that the population in "the end of the world" will be 4 times the currently population

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Aug 17, 2017

Unrigorous method is that as we approach carrying capacity population is constant so set dy/dx=0 and we're done

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