字典翻译 问答 小学 数学 已知sin(5π-θ)+sin(5π/2-θ)=根号7/2求(1)[sin(π/2+θ)]^3-[cos(3π/2-θ)]^3(2))[sin(π/2-θ)]^4-[cos(7π/2+θ)]^4
问题标题:
已知sin(5π-θ)+sin(5π/2-θ)=根号7/2求(1)[sin(π/2+θ)]^3-[cos(3π/2-θ)]^3(2))[sin(π/2-θ)]^4-[cos(7π/2+θ)]^4
问题描述:

已知sin(5π-θ)+sin(5π/2-θ)=根号7/2

求(1)[sin(π/2+θ)]^3-[cos(3π/2-θ)]^3

(2))[sin(π/2-θ)]^4-[cos(7π/2+θ)]^4

邓武回答:
  根据诱导公式,sin(5π-θ)+sin(5π/2-θ)=sinθ+cosθ=根号7/2   (sinθ+cosθ)^2=7/4   (sinθ)^2+(cosθ)^2+2sinθcosθ=7/4   2sinθcosθ=3/4,sinθcosθ=3/8   (1)[sin(π/2+θ)]^3-[cos(3π/2-θ)]^3=(cosθ)^3+(sinθ)^3   =(sinθ+cosθ)[(sinθ)^2-sinθcosθ+(cosθ)^2]=(5倍根号7)/16   (2)[sin(π/2-θ)]^4-[cos(7π/2+θ)]^4=(cosθ)^4-(sinθ)^4   =[(sinθ)^2+(cosθ)^2][(sinθ)^2-(cosθ)^2]   =(sinθ+cosθ)(sinθ-cosθ)   =(sinθ+cosθ)*根号[(sinθ+cosθ)^2-4sinθcosθ]=(根号7)/4
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