simplify: find s_n for th arithmetic series in which a_1 = 3, d= 1/2 and a_n = 15?

Question

a. 225
b. 9
c. 45
d. 210

Answer 1

we're summing from 3 to 15 in increments of 1/2
3 + 3.5 + 4 + 4.5 + 5 + 5.5 + 6 + 6.5 + 7 +7.5 + 8 +8.5... +15
= 18 * 12+ 9
= 225