已知cos(75°+θ)=1/3,θ为第三象限角,求值:cos(-255°-θ)+sin(435°+θ)
已知cos(75°+θ)=1/3,θ为第三象限角,求值:cos(-255°-θ)+sin(435°+θ)
数学人气:452 ℃时间:2020-01-30 02:33:45
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cos(-255°-θ)+sin(435°+θ)=cos(255°+θ)+sin(360°+75°+θ) =cos(180°+75°+θ)+sin(75°+θ) =-cos(75°+θ)+sin(75°+θ) θ为第三象限角,75°+θ为第三或第四象限角 sin(75°+θ)=-根号(1-cos^2[(75°+θ)])=-(2根号2)/3 cos(-255°-θ)+sin(435°+θ)=-1/3-(2根号2)/3
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