The first term of A.Pis 19and the sum of the first eight terms is 136.find common difference and sum of first twentyterms
using the usual formulas, you know that
a = 19
8/2 (2*19 + 7d) = 136
so d = -4/7
finally,
S20 = 20/2 (2*19 + 19(-4/7)) = 271 3/7
unusual, but that's what the math says ...