# How do you find the sum of the arithmetic sequence having the data given a_1= 5, a_n = 3, n = 20?

Feb 6, 2016

The information give gives you the first term, the last term and the number of terms. This means we can use the formula ${S}_{n} = \left(\frac{n}{2}\right) \left(a + {t}_{n}\right)$.

#### Explanation:

${S}_{20} = \left(\frac{20}{2}\right) \left(5 + 3\right)$

${S}_{20} = 10 \left(8\right)$

${S}_{20} = 80$