a1=6 a2=2a1=12 a3=2a2=2^2a1=24 a4=2a3=2^3a1=48 . an=2a(n-1)=2^(n-1) *a1=6*2^(n-1) 规律：6,12,24,48,96,.,6*2^(n-1）,6*2^n,.
