"4th term is 7" ---> a+3d = 7
"7th term is 13" ---> a+6d = 13
subtract them:
3d = 6
d = 2
sub back into my first equation to find a,
then use the Sum of Terms formula to find sum(20)
find the sum to 20 terms of an A.P. whose 4th term is 7 and 7th term is 13
1 answer