已知cos(75°+θ)=1/3,θ为第三象限角,求cos(255°+θ)+sin(435°+θ)
问题描述:
已知cos(75°+θ)=1/3,θ为第三象限角,求cos(255°+θ)+sin(435°+θ)
答
cos(255°+θ)+sin(435°+θ)
=cos(75°+180°+θ)+sin(75°+360°+θ)
=sin(75°+θ)-cos(75°+θ)
θ为第三象限角
cos(75°+θ)=1/3>0
∴75°+θ是第四象限角
∴sin(75°+θ)