a1+a2+a3=3a2=9 a2=3 设公差为d a1=a2-d=3-d a3=a2+d=3+d a1*a2*a3=(3-d)*3(3+d)=15 9-d^2=5 d^2=4 d1=2 d2=-2 d=2 a1=a2-d=1 an=a1+(n-1)d=2n-1 d=-2 a1=a2-d=5 an=a1+(n-1)d=7-2n