證明:2+cosx+sinx*cosx的值恒大於0

證明:2+cosx+sinx*cosx的值恒大於0

2+cosx+sinx*cosx
=2+cosx+1/2sin2x
因為cosx>=-1,sin2x>=-1
所以2+cosx+sinx*cosx>=1/2>0