sinβ=(1-(1/5))^(1/2)=2(√5)/5
tana=-1/3<0 ,a∈(π/2,π)
sina/cosa=-1/3
(sina)^2/(1-(sina)^2)=1/9
sina=(√10)/10
cosa=-3(√10)/10
f(x)=√2sin(x-a)+cos(x+β)
=√2(sinx*cosa-cosx*sina)+(cosx*cosβ-sinx*sinβ)
=-(√5)sinx<=√5
最大值=√5