Mathematica: Printing Numeric Result

1. Jun 12, 2012

Joel_662

Hey,

So im fairly new to Mathematica (and the physics forums too). I am having a problem with mathematica not calculating the numeric result to an very complex integral. My question is, how do I force it to evaluate the integral as a number? I'm assuming that the integral of a square root is possible in mathematica as i checked the help files and it seems to suggest it is. I have tried the evaluation both with and without using n[A] which seems to be the suggested method of forcing a numeric evaluation.

K=1
R=1
VM=1
L=1
z=-0.000000*x^6 - 0.000000*x^5 + 0.000049*x^4 + 0.000115*x^3 - 0.018528*x^2 - 0.033042*x + 2.964166
dzdx=D[z,x]
absdzdx=Abs[dzdx]
sqrtfactor=Sqrt[1+dzdx^2]
A=2*Pi*K*R*VM*L*(Integrate[(6.5+4.56*(absdzdx)+0.654*(absdzdx)^2)*x*sqrtfactor,{x,0,r}])*((2*Pi*\!$$\*SubsuperscriptBox[\(\[Integral]$$, $$0$$, $$r$$]$$\((x*sqrtfactor)$$ \[DifferentialD]x\)\))/10000)

I have tried to break up the calculations so that mathematica doesnt take to long to calculate it but it still seems to take at least 3 or 4 mins of running before it displays an answer. The variables do unfortunately need to stay seperate as they will be changed. I apologise in advance if it is just some stupid error of mine.

2. Jun 13, 2012

Bill Simpson

If you can supply a numeric value for r, perhaps like this

r = 1;

then this

In[1]:= A=2*Pi*K*R*VM*L* NIntegrate[((6.5+4.56*absdzdx+0.654*absdzdx^2)* x*sqrtfactor),{x,0,r}]*
2*Pi/10000*NIntegrate[x*sqrtfactor,{x,0,r}]]

Out[1]= 0.00669876

evaluates in a fraction of a second.

If you cannot provide a numeric value for r then I doubt you will get a satisfactory answer.

Last edited: Jun 13, 2012