Question

i really need to know this please!!!
Find the sum of the first 18 terms of the arithmetic sequence in which the first term is -9 and the 18th term is -60.

Answer 1

\[a = -9\]
\[a_{n} = a + (n-1)d\]
a = first term
n = number of terms
d = difference between two terms
\[a_{18} = -9 + (18-1) d\]
\[-60 = -9 + (17) d\]
Find d
then sum of the series will be
\[\sum_{k=0}^{n-1} (a+kd) = (n/2)(2a + (n-1)d)\]