2(sinπ/5)*(cos3π/5)化简如题

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2(sinπ/5)*(cos3π/5)化简如题2(sinπ/5)*(cos3π/5)化简如题2(sinπ/5)*(cos3π/5)化简如题2(sinπ/5)*(cos3π/5)=-2(sinπ/5)*

2(sinπ/5)*(cos3π/5)化简如题
2(sinπ/5)*(cos3π/5)化简
如题

2(sinπ/5)*(cos3π/5)化简如题
2(sinπ/5)*(cos3π/5)=-2(sinπ/5)*(cos2π/5)=-[2(sinπ/5)*(cosπ/5)*(cos2π/5)]/(cosπ/5)=-[2(sin2π/5)*(cos2π/5)]/(2cosπ/5)=-(sin4π/5)/(2cosπ/5)=-1/2* tanπ/5