向量AC·向量BC
=(3cosx-4)3cosx+3sinx(3sinx-4)
=9cos^2x-12cosx+9sin^2x-12sinx
=9-12(sinx+cosx)
=0
sinx+cosx=9/12=3/4
(sinx+cosx)的平方
=1+2sinxcosx
=1+sin2x=9/16
sin2x=-7/16
(2sinx的平方+sin2x)/(1+tanx)
=(2sixcosx*sin/cox+sin2x)/(1+tanx)
=sin2x(1+tanx)/(1+tanx)
=sin2x
=-7/16