摘要：记u=ax,则 f(x)=u[u-(3a2+1)]=g(u)对称轴为u=(3a2+1)/2,要使f时递增,当0<a<1时u=ax∈(0,1]且递减,只须1≤(3a2+1)/2,解得;当a>1时无解.故选B;

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