tanx=sinx/cosx=3sinx=3cosx同乘以sinxsin^2 x=3sinxcosx.1同乘以cosxsinxcosx=3cos^2x.21*3-2==> 3sin^2x-sinxcosx=9sinxcosx-3cos^2x10sinxcosx=3sinxcosx=3/10答：sinxcosx=3/10