a∈(π/2,π)2a∈(π,2π)tan2a= -4/32a∈(3π/2,2π)tan2a=2tana/(1-tan^2a)-4/3=2tana/(1-tan^2a)tana/(1-tan^2a)=-2/32tan^2a-3tana-2=0(2tana+1)(tana-2)=0tana=-1/2或tana=2（舍去）所以tana=-1/2