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Re: integral that includes symbols
Posted:
Aug 6, 2013 1:50 PM


On 8/6/2013 10:21 AM, Torsten wrote: > "Young " <ryuyr77@gmail.com> wrote in message > <ktqvqm$4lk$1@newscl01ah.mathworks.com>... >> "Torsten" wrote in message <ktqpcf$ga0$1@newscl01ah.mathworks.com>... >> > "Young " <ryuyr77@gmail.com> wrote in message >> <ktql9j$6n2$1@newscl01ah.mathworks.com>... >> > > Hi >> > > > > I have an equation: F=L*cos(theL). L is a constant. Integral >> of F over zero to (1/2)pi should give L. But I could not make that >> answer Please see below my code and help me how to fix this problem. >> I have much more complex equation but simplified my problem to this >> example. > > > > syms L; >> > > theL=[0+0.001:pi/1000:pi/20.001] >> > > F=L*cos(theL); >> > > trapz(theL,F) >> > > > > ans: >> (21828504695225120901251714223243*L)/162259276829213363391578010288128 + >> (634496096455597832693*pi*L)/2305843009213693952000 >> > > If you know the explicit formula for the function you want to >> integrate, then > for symbolic integration use MATLAB's "int", for >> numerical integration use MATLAB's "quad". >> > > Best wishes >> > Torsten. >> >> Hi Torsten >> >> Thanks for your help! But it did not work... Please see below. It >> looks "quad" does not allow to use sym... I need to use numerical >> solution. >>  >> >> syms L; >> y=@(theL) (L*cos(theL)); >> quad(y,0,1/2*pi) >> >> ans: Undefined function 'isfinite' for input arguments of type 'sym'. >> Error in quad (line 79) >> if ~isfinite(y(1)) > > As I wrote above: > If symbolic variables are involved, try "int", else use "quad". > E.g. > > L=1; > y=@(theL) (L*cos(theL)); > quad(y,0,1/2*pi); > > should work as well as > > syms L,theL; > int(L*cos(theL),theL); > > Best wishes > Torsten.
Just correcting a small typo:
syms L theL % no comma in between wt = int(L*cos(theL),theL) or wt = int(L*cos(theL),theL,0,pi/2)
Alan Weiss MATLAB mathematical toolbox documentation



