you have
a = 1
d = 4
So, S15 = 15/2 (2*1 + 14*4)
For (b) , you have
a = -8
d = 5
And as always,
Sn = n/2 (2a + (n-1)d)
Now plug in your numbers
Find the sum given for each of the following series:(a) Sn=1+5+9+....(n=15)
(b) Sn=(-8)+(-3)+2+... (n=20)
2 answers
(a) the common difference is 4 ... the 15th term is ... 1 + [4 * (15 - 1)] = 57
... S15 = (1 + 57) * (15 / 2)
(b) the common difference is 5 ... the 20th term is ... -8 + [5 * (20 - 1)] = 87
... S20 = (-8 + 87) * (20 / 2)
... S15 = (1 + 57) * (15 / 2)
(b) the common difference is 5 ... the 20th term is ... -8 + [5 * (20 - 1)] = 87
... S20 = (-8 + 87) * (20 / 2)
1 answer