19.已知sin(+2α)sin(-2α)=,α∈(,),求:2sin2α+tanα-cotα-1的值.
20.已知△ABC的面积为3,且满足0≤\s\up6(→(→)·\s\up6(→(→)≤6,设〈\s\up6(→(→),\s\up6(→(→)〉=θ.
(1)求θ的取值范围;
(2)求函数f(θ)=2sin2(+θ)-cos2θ的最大值与最小值.
答案与解析
1.A sin(α+β)sin(α-β)=-(cos2α-cos2β)
=-[(2cos2α-1)-(2cos2β-1)]=-(cos2α-cos2β)=-m.