Sn=1/4n^2+2/3n+3Sn-1=1/4(n-1)^2+2/3(n-1)+3An=Sn-Sn-1=1/4n^2+2/3n+3-(1/4(n-1)^2+2/3(n-1)+3)=n/2+5/12希望能解决您的问题。
