数列0.5,0.55,0.555,0.5555,……的前n项之和为多少
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![数列0.5,0.55,0.555,0.5555,……的前n项之和为多少](/uploads/image/z/125975-47-5.jpg?t=%E6%95%B0%E5%88%970.5%2C0.55%2C0.555%2C0.5555%2C%E2%80%A6%E2%80%A6%E7%9A%84%E5%89%8Dn%E9%A1%B9%E4%B9%8B%E5%92%8C%E4%B8%BA%E5%A4%9A%E5%B0%91)
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数列0.5,0.55,0.555,0.5555,……的前n项之和为多少
数列0.5,0.55,0.555,0.5555,……的前n项之和为多少
数列0.5,0.55,0.555,0.5555,……的前n项之和为多少
2 1 n n
Sn=5/9[(1/ 10) (10 -1)+…(1/10) (10 -1)]
2 n
=5 /9[1-(1/10 ) +1 -(1/10) +…+1-(1/ 10) ]
...
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2 1 n n
Sn=5/9[(1/ 10) (10 -1)+…(1/10) (10 -1)]
2 n
=5 /9[1-(1/10 ) +1 -(1/10) +…+1-(1/ 10) ]
n
=5 /9[n +(1-10 )/ 90]
不好打,化简自己来吧。
收起
n*5*10-1 +(n-1)*5*10-2 +.....+1*5*10 -n
第n项是 5*(0.1)^n
Sn =(0.5+5*(0.1)^n)* n/ 2
= 5.5n /2