Question

In a arithmetic sequence 9th term is 19 and the sum of the first 12 term is 168.find the sum of first eight term.

Answer 1

a+(n-1)d formula

sum=n/2(a+l)

n/2(a+l)=168

a=a  ;  l=a+11d

solve it for n and sub those in a+(n-1)d to get the answer