The syntax is: SEQ(term, variable, start value, end value, step)
Store the sequence in a list(i.e. in L1 = 2ND 1), otherwise you can't use the values anymore and you have to type these commands over and over again.
To calculate series you can use the command cumSum( which you find at 2ND STAT(= LIST) OPS 6. Store the results of this operation in a list too.
I've attached the screen shots of
To change from decimal numbers to fractions (which I prefer) use MATH Frac ENTER. The last result will be transformed into a fraction - if it is possible.