Question
If $ \sin^{-1} a $ is the acute angle between the curves $ x^2 + y^2 = 4x $ and $ x^2 + y^2 = 8 $ at (2, 2), then $ a $ = _______.
If $ \sin^{-1} a $ is the acute angle between the curves $ x^2 + y^2 = 4x $ and $ x^2 + y^2 = 8 $ at (2, 2), then $ a $ = _______.