等差数列a3=a1+2d,a2=a1+d,a4=a1+3d,s2=2a1+d,s3=3a1+3d,s4=4a1+6dS3^2=9S2,S4=4S2,(3a1+3d)²=9(2a1+d),a1²+2a1d+d²=2a1+d ①4a1+6d=4(2a1+d),d=2a1 ②联立,解得：a1=4/9,d=8/9数列的通项公式an=8/9...