a3+a6+a9=(a+2d)+(a+5d)+(a+8d)=3a+15d=12 得a+5d=4(a+2d)(a+5d)(a+8d)=28因为a+5d=4所以(a+2d)(a+8d)=7a^2+10ad+16d^2=7(4-5d)^2+10(4-5d)d+16d^2=7化简得d^2=1因为数列递增,所以d>0 所以d=1从而得a=-1所以an=-1+(n...