if 10th term of ap is 21, find the sum of first 10 terms
Answers
Answered by
1
a
10
=21
a+(10−1)d=21
a+9d=21....(1)
S
10
=120
2
10
[2a+(10−1)]d=120
2a+9d=24......(2)
solving (1) and (2), we get,
a=3
from (2), we have,
d=2
a
n
=a+(n−1)d
a
n
=3+(n−1)2
∴a
n
=2n+1
Similar questions