>>348
　sinα = 1/3,
　sinβ = 7/9,
とおく。（0<α、β<π/2)
　cosα = (2/3)√2,
　cosβ = (4/9)√2,

∴　cos(右辺) = cos(α+β)
　= cosα･cosβ - sinα･sinβ
　= (2/3)(4/9)･2 - (1/3)(7/9)
　= 16/27 - 7/27
　= 1/3,
スッキリ。