1/1x4十1/4x7十---十1/(3n一2)x(3n十1)求和

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1/1x4十1/4x7十---十1/(3n一2)x(3n十1)
=1/3(1/1-1/4+1/4-1/7+···+1/(3n-2)-1/(3n+1))
=1/2(1-1/(3n+1))
=1/2×3n/(3n+1)
=3n/(6n+6)

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