已知角a为钝角,且sin(a+π/12)=1/3,则cos(a+5π/12)=?
问题描述:
已知角a为钝角,且sin(a+π/12)=1/3,则cos(a+5π/12)=?
答
角a为钝角,且sin(a+π/12)=1/3,则a+π/12为钝角,于是cosa(a+π/12)=-(2根2)/3 a+5π/12=a+π/12+π/3cos(a+5π/12)=cos(a+π/12)cosπ/3-sin(a+π/12)sinπ/3=-[2(根号2)/3](1/2)-(1/3)(根号3)/2=-(...