Question

which term of the AP 5, 9, 13, 17, andalso find the sum 5 + 9 + 13 + 17+ ----81​

Answer 1

sum

an=a+(n-1)d

81=5+(n-1)4

81=5+4n-4

81=4n+1

4n=80

n=20

Sn= n/2(a+n)

Sn=20/2(5+81)

=10(86)

Sn=860

HOPE IT HELPS U.....