公比为2,首项为1,n+1项的等比数列求和：1+2+4+8+…2^n=1*[1-2^(n+1)]/(1-2)=2^(n+1)-1
则limn→∞(1+2+4+8+…2^n)/2^n
=limn→∞[2^(n+1)-1]/2^n
=limn→∞(2-1/2^n)
=2
limn→∞(1+2+4+8+…2^n)/2^n
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