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∵等差數列的前n項和為25,前2n項和為100,∴Sn,S2n-Sn,S3n-S2n成等比數列,∴25100-25,S3n-100成等差數列,∴2(100-25)=25+(S3n-100),解得S3n=225.故答案為:225.
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