To have the summation use sum and a usual with have lower bound and upper bound. For example if you want `sum_(n=2)^10` the lower bound is 2 and use sum_(n=2) and for upper bound which is 10 here use ^10. If the upper bound is not a number and it is more than one character then use parenthesis ^(2n).
For infinity use 2 lower case letter 'o' as oo
sum_(n=0)^oo which shows as `sum_(n=0)^oo`
putting them together to have Taylor series here is how you put them all