The sum of the first 18 terms of an arithmetic series is 576. The common difference is -8. Determine the value of the first term, t1.
+36915 S = (n/2) × (2a + (n−1)d) substituting in what is given
576 = (18/2)(2a + (18-1)(-8)
9 (2a -136)
576 = 18a - 1224
a1 = 100
100 + 10(-8) = 20 = a11
