tana=sina/cosa=2
(sina)^2+(cosa)^2=1
解得sina=(2*5^1/2)/5,cosa=(5^1/2)/5或sina=-(2*5^1/2)/5,cosa=-(5^1/2)/5
所以sin^2a=4/5,sina*cosa=1/5
所以原式=4/5-4*1/5+2=2