Question

已知{an}为等差数列，Sn为其前n项和。若a1=1/2，S2=a3，则a2=？

Answer 1

S2=a1+a2所以a1+a2=a3 设等差数列的公差为x 则a3=a2+x 即a1+a2=a2+x 推出x=a1=1/2 a2=a1+x=1

Answer 2

S2=a1+a2=a1+a1+d=2a1+d=a3=a1+2d
∴a1=d=1/2
∴a2=a1+d=1

Answer 3

1