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可以把它拆開就變成:(2+4+8+.+2n)-(3x5^-1+3x5^-2+.+3x5^-n)可以寫成兩種不同的等比數列 又因為2可以寫成2^1,4也可以寫成2^2,即是Sn1=2x(1-2^n)/1-2,Sn2=3x5^-1x(1-0.2^n)/1-0.2 所以Sn=Sn1-Sn2
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