23.
(1)a1=s1=1-1/2=1/2
s2=1-1/2的平方=3/4
s2=a1+a2 a2=3/4-1/2=1/4
s3=1-1/2的立方=7/8
s3=a1+a2+a3=7/8
a3=7/8-1/4-1/2=1/8
(2)an=sn-s(n-1)=(1-1/2的n次方)-(1-1/2的n-1次方)=1/2的n次方
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