已知数列{an}的前n项和为Sn,给出下列四个命题: ①若Sn=n2+bn+c(b,c∈R),则{an}为等差数列; ②若{an}为等差数列且a1>0,则数列{a1an}为等比数列; ③若{an}为等比数列,则{lgan}为等差数列; ④若{an}为等差数列,且Sn=100,a2n+1+a2n+2+…+a3n=-120,则S2n=90,其中真命题有_____.