Solution: The original title is cos (a+b) =1/5cos (a-b) = 3/5 to find tanatanb.

Cosa Cosb = 2/5 from Cosa Cosb-Sina sinb = 1/5 Cosa Cosb+Sina sinb = 3/5。

Sina sinb =- 1/5 ∴tanatanb=sinasinb/cosacosb=- 1/2

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