Re: integrate problem

*To*: mathgroup at smc.vnet.net*Subject*: [mg98872] Re: [mg98837] integrate problem*From*: Andrzej Kozlowski <akoz at mimuw.edu.pl>*Date*: Mon, 20 Apr 2009 19:11:46 -0400 (EDT)*References*: <200904200939.FAA12325@smc.vnet.net>

On 20 Apr 2009, at 18:39, sean_incali at yahoo.com wrote: > I'm having some issues understanding this... Following works as > expected. > > s is the variable in time. a is a parameter. > > Integrate[1/(s + a), s] > > gives Log[a + s] > > But the following gives the integral itself. > > Integrate[1/(s[t] + a), t] > > If the function that needs to be integrated w.r.t time is 1/(s[t] + > a), how do I accomplish that? You can't accomplish that unless you give Mathematica an explicit expression for s[t] (and you will have to be lucky then too). How could you possible expect this to work in general? If this did, you could integrate practically any function f[t] by using a suitable s[t]. This has nothing to do with Mathematica, its just basic mathematics. > It seems like if integrate it wrt to s > [t] it will work. > > Integrate[1/(s[t] + a), s[t]] > > > But does that sound about right? Right? In what sense "right"? It's a completely different thing from your first case. Here you might as well write t (or s) instead of s[t] as it has nothing whatever to do with s[t]. Note that this also gives an answer: Integrate[s'[t]/(a + s[t]), t] Log[a+s[t] ] but this also has nothing to do with your original integral... Andrzej Kozlowski

**References**:**integrate problem***From:*sean_incali@yahoo.com