令B = π / 4.-> tan(A + B) = (tanA - tanB )/(1+ tanA tanB) = 1 / 5 ->( tanA - 1 )/ (1+ tan A) =1 / 5-> tan A = 3 / 2.（sin2A-sin^2A)/(1-COS2a) =( 2cosA sin A - sin^2(A) ) / ( 1 - (1 - 2sin^2(A) ) ) = ...