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in (e.g. secret); with (e.g. success)
「裏」 has another reading with different meanings.
二つの事柄は表裏一体の関係だ。
There is an inseparable relationship.
裏面を確認してください。
Please check the back side.
裏工作が行われていたらしい。
It seems behind-the-scenes maneuvering was going on.