am=a1+(m-1)d
am≥n
a1+(m-1)d≥n
∵m有最小值
∴d>0
m≥(n+d-a1)/d
(n+d-a1)/d=3n-2
n=(3d-a1)/(3d-1)
(3d-a1)/(3d-1)≥1
3d-a1≥3d-1
a1≤1