The first term of an arithmetic sequence is -8. Find the 20th term if the sum of the first 20 terms is 790.
Sum, S, of the first 20 terms is given by
S = ( a1 + a20) * (n /2)
790 = (-8 + a20) * (20/2)
790 = (-8 + a20) * 10 divide both sides by 10
79 = -8 + a20 add 8 to both sides
87 = a20
