3.(2021•甘肃模拟)数列{an}\{a_{n}\}{an}的前nnn项和为SnS_{n}Sn,若点(n,Sn)(n,S_{n})(n,Sn)在函数f(x)=x2+2xf(x)=x^{2}+2xf(x)=x2+2x的图象上,则a2021=(a_{2021}=(a2021=( )))