1. 5.4 Sum of Arithmetic Series (1/4)
Sn = (a + l) n 2
Sum (add)
to n terms
Last
(or nth Term)
Term
Position
First
Term
a+(n-1)d
Sn = n/2(a + a+(n-1)d)
Sn = [2a+(n-1)d] n 2

2. 5.4 Sum of Arithmetic Series (2/4)
Sn = (a + l) n 2
Use
If you know or are asked to find ‘l’
Use
Sn = [2a+(n-1)d] n 2
If you don’t know and are not asked to find ‘l’

3. 5.4 Sum of Arithmetic Series (3/4)
Sum the first 30 terms of the series …
a = 12
n = 30
d = 2
Sn = n/2(2a +(n-1)d)
S30 = 30/2x(2x12 +(30-1)x2)
= 1260

4. 5.4 Sum of Arithmetic Series (4/4)
Sum the series
… + 100
Find n.
a = 8
d = 4
n = 24
l = 100
a = 8
d = 4
Sn = n/2(a + l)
Tn = a +(n-1)d
S24 = 24/2( )
100 = 8 +(n-1)x4
100 = 8 + 4n - 4
100 = 4n + 4
= 1296
4n = 96
n = 24