两边除以2^nan/2^n=4a(n-1)/2^n+1an/2^n=2a(n-1)/2^(n-1)+1an/2^n+1=2a(n-1)/2^(n-1)+2=2[a(n-1)/2^(n-1)+1]所以an/2^n+1是等比，q=2则an/2^n+1=(a1/2^1+1)*2^(n-1)=3/4*2^n所以an=2^n*(-1+3/4*2^n)即an=-2^n+3/4*4^n
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