Question

how many terms of AP 2, 7, 12, 17....... add upto 990

Answer 1

990=n/2[2*2 + (n-1)5]
1980=n(4+5n-5)
1980=5n2 -n
5n2-n-1980=0
5n2-(100-99)n-1980=0
5n2-100n+99n-1980=0
5n(n-25)+99(n-25) =0
(n-25)(n+99)=0
n=-99(neglect)
n=25
Sn25=990
Sn26=upto 990