Why did they choose x for unknown?

Question

anonymous · Answer

You want to find the 4th derivative of $f(x)=\ln(2x+1)(3^{x+2})+x^3$?

anonymous · Answer

Yeah

anonymous · Answer

You have to find it one by one. Find the first, second until the fourth derivative.

anonymous · Answer

Really D:

anonymous · Answer

You don't need to work on the last term if that might help, since its fourth derivative is zero.

across · Answer

This is a separable (and already separated), linear ODE; you can solve it by simple integration. Remember that you'll end up with 4 integrating constants.

anonymous · Answer

Thanks

across · Answer

Btw, you have to work on every term since we're integrating.

anonymous · Answer

@across: I think he wants to find the fourth derivative of f(x) that I wrote above. I thought its a DE at first, but he says it's not!

across · Answer

Oh, I just noticed that, at first, he asked for a solution to that ODE, then a post later he wants to find the fourth derivative of said function.

o_o

across · Answer

In any case, I'll leave this here; perhaps it'll end up being helpful:
$$\frac{dy}{dx}=0,$$$$y=c.$$$$\frac{d^4y}{dx^4}=0,$$$$y=\frac{1}{6}c_1x^3+\frac{1}{2}c_2x^2+c_3x+c_4.$$