JAMB - Mathematics (2005 - No. 17)
Find the sum of the first 20 terms of the series 8, 12, 16, ....., 96
1400
1040
960
920
Forklaring
8, 12, 16, .....96
a = 8, d = 4, l = 96, n = 20
\(S_{20} = \frac{n}{2}(2a + (n-1)d\)
\(S_{20} = \frac{20}{2}((2\times 8) + (20-1)\times 4)\)
\(S_{20} = 10(16 + (19\times 4)) = 10 \times 92\)
=920