a(n+1)=2^n*ana(n+1)/an=2^n累乘得a2/a1*a3/a2*.....*a(n+1)/an=2^(1+2+...+n)a(n+1)/a1=2^(1+2+...+n)=2^[n(1+n)/2]a(n+1)=2^(1+2+...+n)=2^[n(1+n)/2]an=2^[n(n-1)/2]
