Question

the third term of an arithmetic sequence is -12 and the seventh term is 8.what is the sum of the first 10 terms

Answer 1

According to equation , a+2d= -12...(1), a+7d=8 ...(2)

subtract equation 1and 2

so, d= 5 ,a=-22

now we know that , sum formula s10=n/2(2a+(n-1 )d)

s10 =10/2(2(-22)+(10-1)5)

=5(-44+(9)5)

=5(-44+45)

=5(1)=5