LaRoche finished 2-for-3 while Steve Lombardozzi added a pair of RBI with a run-scoring single and a sacrifice fly for the Nats, who clinched their first National League East crown Monday.
Zach Duke (1-0) earned his first win with a scoreless fifth inning and Drew Storen garnered his fourth save as Washington evened its three-game season-ending series.
Phillies reliever Josh Lindbom (3-5) was saddled with the loss.
Darin Ruf socked a pair of solo homers for Philadelphia.
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