# Given the first and last term, a = 6, t9 = 24, how do you find the sum of the arithmetic series?

Formula is ${S}_{n} = n \frac{2 a + \left(n - 1\right) d}{2}$
${t}_{n} = a + \left(n - 1\right) d$
So ${S}_{n} = n \frac{a + l}{2}$