sn=(2-3x5^-1)+(4-3x5^-2)+…+(2n-3x5^-n)要详解答案,急用!
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sn=(2-3x5^-1)+(4-3x5^-2)+…+(2n-3x5^-n)要详解答案,急用!
sn=(2-3x5^-1)+(4-3x5^-2)+…+(2n-3x5^-n)要详解答案,急用!
sn=(2-3x5^-1)+(4-3x5^-2)+…+(2n-3x5^-n)要详解答案,急用!
sn=(2-3x5^-1)+(4-3x5^-2)+…+(2n-3x5^-n)
=(2+4+.+2n)-(3x5^-1+3x5^-2+.+3x5^-n)
=(2^1+2^2+.+2^n)-(3x5^-1+3x5^-2+.+3x5^-n)
=2x(1-2^n)/(1-2)+3x5^-1x(1-0.2^n)/(1-0.2)
满了
(2+4+...+2n)-3(5^-1+5^-2+...+5^-n)=n(1+n)-3*(1-5^-n)/4,前一个括号用等差数列前n项和公式,首项是2,公差是2,后一个括号用等比数列前n项和公式,首项是1/5,公比是1/5