a·b+1=0.cosθcos5θ+sinθsin5θ=cos4θ=-1,sin4θ=0
(sin2θ+cos2θ)²=1+sin4θ=1,sin2θ+cos2θ=±1
[也可:cos4θ=-1,4θ=(2k+1)π.2θ=kπ+π/2,cos2θ=0,sin2θ=±1
sin2θ+cos2θ=±1]
a·b+1=0.cosθcos5θ+sinθsin5θ=cos4θ=-1,sin4θ=0
(sin2θ+cos2θ)²=1+sin4θ=1,sin2θ+cos2θ=±1
[也可:cos4θ=-1,4θ=(2k+1)π.2θ=kπ+π/2,cos2θ=0,sin2θ=±1
sin2θ+cos2θ=±1]