November 21st, 2016, 12:50 AM  #1 
Senior Member Joined: Dec 2014 From: The Asymptote Posts: 142 Thanks: 6 Math Focus: Certainty  Differential Equations
I require some clarification please, Given $\displaystyle y' = y^2$ Verify that all members of $\displaystyle y = \frac{1}{x+C}$* are solutions of $\displaystyle y'$ $\displaystyle y' = \frac{dy}{dx} = \frac{d}{dx}\bigg(\frac{1}{x+C}\bigg) = \frac{1}{(x+C)^2}$ 1) $\displaystyle y = \frac{1}{x+C}$ 2) Substituting $\displaystyle y$ into $\displaystyle y'$ $\displaystyle y' = y^2 = \bigg(\frac{1}{x+C}\bigg)^2 = \frac{1}{(x+C)^2}$ $\displaystyle y' = y^2$ holds true therefore $\displaystyle y$ is a solution of $\displaystyle y'$ I'm then asked to find a solution of $\displaystyle y' = y^2$ that is not a family member of $\displaystyle y$* ???? Lastly, a solution of the initial value problem: $\displaystyle y' = y^2$ $\displaystyle y(0) = 0.5$ The solution is $\displaystyle y = \frac{1}{x+2}$… in application this could be the vertical displacement of an oscillating spring at time t = 0???? 
November 21st, 2016, 01:13 AM  #2 
Global Moderator Joined: Dec 2006 Posts: 19,718 Thanks: 1806 
$y = \dfrac{1}{x+C}\implies y' = \dfrac{1}{(x+C)^2}$, so $y' = y^2$ is satisfied. Another solution of $y' = y^2$ is $y = 0$, which is where you're from. On reaching the end of the question, why did you mention an oscillating spring, which seems to have no relevance or connection? 
November 21st, 2016, 07:37 AM  #3 
Senior Member Joined: Mar 2015 From: New Jersey Posts: 1,546 Thanks: 110 
Very interesting thread. Thanks 1) $\displaystyle \frac{\mathrm{d} y}{\mathrm{d} x}=y^{2}$ 2) $\displaystyle \frac{\mathrm{d^{2}y} }{\mathrm{d} x^{2}}=2y$, harmonic oscillator 3) $\displaystyle \frac{\mathrm{d} (\frac{\mathrm{d} y}{\mathrm{d} x}y+y^{2})}{\mathrm{d} x}=0$ By 3), a solution of 2) satisfies 1). Solution of 2): 4) y=Acoswx+Bsinwx, w$\displaystyle ^{2}$=2 But I can't make 4) a solution of 1) ? 
November 21st, 2016, 07:50 AM  #4 
Math Team Joined: May 2013 From: The Astral plane Posts: 1,889 Thanks: 769 Math Focus: Wibbly wobbly timeywimey stuff.  
November 21st, 2016, 08:34 AM  #5  
Senior Member Joined: Mar 2015 From: New Jersey Posts: 1,546 Thanks: 110  Quote:
The thanks was a painful one, since I visited to correct myself. EDIT: Actually, y''+2yy'=0 is solvable: d(y')=2ydy y'=y^{2}+C etc REF: y"+yy'=0 Last edited by zylo; November 21st, 2016 at 08:58 AM.  
November 21st, 2016, 08:56 AM  #6  
Math Team Joined: Dec 2013 From: Colombia Posts: 7,445 Thanks: 2499 Math Focus: Mainly analysis and algebra  Quote:
$y'+y^2=0$ is separable and thus completely solvable. The $y=0$ solution appears separately because we divide by $y^2$ in separating the variables, thus denying $y=0$. We thus have to treat that case separately. The direction field will confirm that there are no other solutions. Last edited by v8archie; November 21st, 2016 at 09:05 AM.  
November 21st, 2016, 09:16 AM  #7  
Senior Member Joined: Mar 2015 From: New Jersey Posts: 1,546 Thanks: 110  Quote:
A solution to y'=y$\displaystyle ^{2}$ is not unique since it is nonlinear.  
November 21st, 2016, 09:35 AM  #8 
Math Team Joined: Dec 2013 From: Colombia Posts: 7,445 Thanks: 2499 Math Focus: Mainly analysis and algebra 
You do realise that 1, 2 and 3 are all just derivatives and integrals of each other? They are essentially the same equation.

November 21st, 2016, 09:53 AM  #9 
Global Moderator Joined: Dec 2006 Posts: 19,718 Thanks: 1806  Nonsense. If one is given that $y' = y^2$ for all values of $x$, $y = \dfrac{1}{x+C}$ isn't a solution, as it doesn't define $y$ when $x = C$, but $y = 0$ is a solution (the only, and therefore unique, solution).

November 21st, 2016, 10:28 AM  #10 
Senior Member Joined: Mar 2015 From: New Jersey Posts: 1,546 Thanks: 110  

Tags 
differential, equations 
Thread Tools  
Display Modes  

Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Differential equations  Mh430  Calculus  3  March 15th, 2015 03:22 PM 
Partial Differential Differential Equations  rishav.roy10  Differential Equations  0  August 21st, 2013 05:59 AM 
differential equations  thebigbeast  Differential Equations  2  September 11th, 2012 09:27 AM 
Differential Equations  aaronmath  Differential Equations  0  October 30th, 2011 09:39 PM 
differential equations  skyhighmaths  Differential Equations  3  September 13th, 2011 08:55 AM 