cos2x/(√2cos(x+π/4))=1/5,0

cos2x/(√2cos(x+π/4))=1/5,0

(cos²x-sin²x)/[√2(cosxcosπ/4-sinxsinπ/4)]=1/5
(cosx+sinx)(cosx-sinx)/(cosx-sinx)=1/5
cosx+sinx=1/5
cosx=1/5-sinx
平方
cos²x=1-sin²x=sin²x-2/5*sinx+1/25
由x范围
sinx>0
所以sinx=4/5,cosx=-3/5
tanx=sinx/cosx=-4/3