I'm rusty in ODEs, so this might be simple..

Solve $$f'(x)=f(x)^{2}+4$$

I was able to make a few observations but they don't seem too helpful. First, since $f(x)^{2}+4\geq 4$, we have $f'(x)\geq 4$ everywhere, so in particular $f(x)$ is increasing (this rules out constant functions, for example).

I next considered linear functions. Supposing that $f(x)=ax+b$, then $f'(x)=a$ so by the given condition we must have $$a=(ax+b)^{2}+4$$ for all $x$. In particular, setting $x=1$ gives $$a=(a+b)^{2}+4$$ and setting $x=-1$ gives $$a=(a-b)^{2}+4$$ In particular this means $a+b=a-b$ so that $b=0$. In this case setting $x=0$ gives $a=4$ so our function would have to be $f(x)=4x$, but this clearly does not satisfy the desired condition. Therefore we can rule out linear functions.

I feel like I'm approaching this the wrong way because I'm only ruling out specific classes of functions, rather than proving any properties that $f$ must have.

Motivation: I am preparing for technical interviews and found this problem here.


Your DE is $y'=y^2+4$, which is a separable equation, meaning we can "move" all the $y$ terms to one side and the $x$ terms on the other, as follows: $$\frac{dy}{dx}=y^2+4 \implies \frac{1}{y^2+4}dy = dx$$ Integrating yields $$\int \frac{1}{y^2+4}dy = \int 1dx \implies \frac 12 \arctan(\frac y2)=x+C$$ and so you can solve for $y$: $$y=2\tan(2x+C)$$

  • 1
    $\begingroup$ Great, thanks! I definitely remember seeing this method but haven't had to use it in 10 years or so :) $\endgroup$ – pwerth Mar 4 at 22:35

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