# Find the sum of the arithmetic sequence having the data given a_1=5, a_n=3, n = 20

Find the sum of the arithmetic sequence having the data given ${a}_{1}=5$, ${a}_{n}=3$, n = 20
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The information give gives you the first term, the last term and the number of terms. This means we can use the formula ${S}_{n}=\left(\frac{n}{2}\right)\left(a+{t}_{n}\right)$.
${S}_{20}=\left(\frac{20}{2}\right)\left(5+3\right)$
${S}_{20}=10\left(8\right)$
${S}_{20}=80$