Find
the sum of the 1st 20 terms of the following arithmetic progression:
5+11+17+…………………
Solution
d=6,
A1 = 5,n=20
Sn
= n[2A1 + (n-1)d]
2
S16
= 20[2(5) + (16-1)d]
2
S16
= 10[10 + 15d]
S16
= 10[10 + 15(6)] since d=6
S16
= 10[10 + 90]
S16
= 10 x 900
S16
= 9000
Therefore the sum of 1st 20 terms is 9000.
TRY THIS......
Find
the sum of the 1st 18 terms of the following arithmetic progression:
6+14+22+…………………
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