利用两角和(差)大的余弦公式化简
cos58°cos37°+sin58°sin37°
cos(60°+)-cos(60°-θ)
cos(60°+θ)+cos(60°-θ)
利用两角和(差)大的余弦公式化简
cos58°cos37°+sin58°sin37°
cos(60°+)-cos(60°-θ)
cos(60°+θ)+cos(60°-θ)
1.cos(58-37)=cos21
2.cos60cosθ-sin60sinθ-(cos60cosθ+sin60sinθ)=-2sin60sinθ
3.cos60cosθ-sin60sinθ+cos60cosθ+sin60sinθ=2cos60cosθ
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