只观察-π到π这个区间∵θ是第四象限角,而sin(θ+π/4)>0,∴θ+π/4是第一象限角那麼θ∈(-π/4,0)∴θ-π/4∈(-π/2,-π/4),即tan(θ-π/4)<0又∵tan(θ-π/4)=tan(θ+π/4-π/2)=-cot(θ+π/4)而sin(θ+π/4)=3/5,∴cot(θ+π/4)=4/3∴原式=-4/3
