sum command...

[webmasterpdx]

When I try to evaluate sum(1/x^2,x,1,n), if n = infinity, it evaluates correctly to pi^2/6, but when i put n = 100, 1000, 2000, it calculates it as these giant fractions, which I have to run approx() on to get the decimal value. Then, when I try n = 5000, it runs out of space for the numerator and denominator, and gives me an answer of undef, instead of trying to calculate the decimal answer....
Wondering why it's trying to evaluate this as a huge fraction instead of decimally? and why it's coming up with an answer of undef instead of calculating the correct value?
Thx
-Donald