a,b,a+b成等差数列得,2b=a+(a+b),即,b=2a
a,b,4ab成等比数列得,b^2=4a^2*b,即,b=4a^2
从而,a=1/2,b=1
cn=1/2^n,用等比数列求和公式得S=1-1/2^n