设数列9,99,999,9999,…的前n项和为______.
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设数列9,99,999,9999,…的前n项和为______.
设数列9,99,999,9999,…为数列{an},其前n项和为sn,
∴an=10n-1,
sn=(101+102+…+10n)-n=
10(1−10n)
1−10-n=
10n+1−9n−10
9.
故答案为:
10n+1−9n−10
9.
∴an=10n-1,
sn=(101+102+…+10n)-n=
10(1−10n)
1−10-n=
10n+1−9n−10
9.
故答案为:
10n+1−9n−10
9.
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