Question: Find the 8th and 13th term of the finite geometric series: -1+2+(-4)+8+(-16)+...a20
A geometric sequence is given by
an = a1*r^(n-1)
where an is the nth term,
a1 is the 1st term,
r is the common ratio.
Looking at your series: -1+2+(-4)+8+(-16)+...a20
a1 = -1 and r = -2
8th term
a8 = a1*r^(8-1) = (-1)*(-2)^7 = (-1)*(-128)
a8 = 128
13th term
a13 = a1*r^(13-1) = (-1)*(-2)^(12) = (-1)*4096
a13 = -4096
p.s. if you wish to clarify that you are using a subscript form, then use an underscore.
e.g. write a20 as a_20
If the context of the text you are writing about is sufficiently clear about the use of subscripts then a20 is fine.