Question

等差数列{an}中,a3=3,前7项和S7=28.求数列的公差d

Answer 1

设首项为a 公差为b

则 a3=a+2b

an=a+（n-1）b

Sn=（a1+an）*n/2

S7=（a1+a1+6b）*7/2=28

则 2（a1+2b）+2b=8

2*3+2b=8

则 b=1