For an arithmetic sequence where a1 = 13 and the common difference is 3, find s7. A. 162 B. 157 C. 154 D. 151

sn=n/2[2a+(n-1)d]
s7 means n=7

a7=7/2[2(13)+(7-1)(3)]=7/2(44)=154

Ans:C