1α是第二象限2kπ+(π/2)<α<2kπ+πkπ+(π/4)<α/2√1-cos²(α/2)=-sin(α/2)2kπ+(5π/4)<α/2<2kπ+(3π/2)α/2在第三象限2θ是第二象限sin(cosθ)<0,cos(sinθ)>0P(sin(cosθ),cos(sinθ))在第二象限3.cos100°=k<0cos80°=-ksin80°=√1-k²tan80°=(√1-k²)/k
