Question

、已知{an}为等差数列,Sn为其前n项的和,若a1=1/2,S2=a3,则a2=____,Sn=_____

Answer 1

S2=a1+a2=a1+a1+d=2a1+d=a3=a1+2d

∴a1=d=1/2

∴a2=a1+d=1

Sn=na1+n(n-1)d/2=n/2+n(n-1)/4=n(n+1)/4