sinx+cosx=7/13
(sinx+cosx)^2=(7/13)^2
(sinx)^2+2sinxcosx+(cosx)^2=49/169
sinxcosx=-60/169
由于0＜x＜π,sinx>0,cosx<0,tanx=sinx/cosx<0
[(sinx)^2+(cox)^2]/sinxcosx
=sinx/cosx+cosx/sinx
=tanx+1/tanx
=1/(-60/169)
令tanx=t
t+1/t=-169/60
60t^2+169t+60=0
由于t<0,t=-√(b^2-4ac)/2a=-√(169*169-4*60*60)/2*60=-7√289/120