a = 1; for(i=2; i<=n; i++) { a = (a+9/a)/2 }
a(n)   if I know initial conditions and recurrence function F

Introduce n and click
n  
To change initial conditions and F open the Code and modify F(n)

Save by clicking here (with the right mouse button) the file on your computer
(change the name if you want, but leave ".htm").
At this point you can use it to calculate any sum.
Just open the source code, edit F(n), and run "save".
You can repeat this several times. 
                              Functions:
abs(a)     / |a|                            log(a)   / log of a base e
acos(a)    / arc cosine of a                max(a,b)
asin(a)    / arc sine of a                  min(a,b)
atan(a)    / arc tangent of a               pow(a,b) / a to the power b
atan2(a,b) / arc tangent of a/b             random() / random n. in [0,1)
ceil(a)    / integer closest to a not < a   round(a) / integer closest to a 
cos(a)     / cosine of a                    sin(a)   / sine of a
exp(a)     / exponential of a               sqrt(a)  / square root of a
floor(a)   / integer closest to a not > a   tan(a)   / tangent of a
PI         / π
examples