cosx+2sinx=-根号5 1+4sinxcosx+3sin^2x=5 2sin2x+3/2(1-cos2x)=4 4sin2x-3cos2x=5 8tanx/(1+tan^2x)-3*(1-tan^2x)/(1+tan^2x)=5 2(tan^2x-4tanx+4)=0 (tanx-2)^2=0 tanx=2