2(sinπ/5)*(cos3π/5)化简

题目

2(sinπ/5)*(cos3π/5)化简
如题

答案

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