How to solve exponential distribution equation using symbolic Matlab?

I have exponential distribution function - F(tau) = beta * e^(-beta*tau). I need to integrate tau*F(tau) over the intervall 0 to inf, My answer should be 1/beta, however I don't get this answer. This is what I did:

>> syms tau beta

>> home

>> syms tau beta

>> f = tau*beta*exp(-beta*tau);

>> int(f, tau)

ans =

-(exp(-beta*tau)*(beta*tau + 1))/beta

Thanks for help, in advance.

[Sushil]

I tried it again and I got the answer:

>> f = x*a*exp(-a*x);

>> assume(a>=0);

>> int(f,x,0, inf)

ans =

1/a

Answer 1

[Bjorn Gustavsson]

It helps the int if you explicitly tells what the range to integrate over is:

int(f,0,inf)

HTH