已知cos(a-π/3)=15/17,a为钝角,求cosa

问题描述:

已知cos(a-π/3)=15/17,a为钝角,求cosa

π>a-π/3>0
sin(a-π/3)=8/17
cosa=cos(a-π/3)cosπ/3+sin(a-π/3)sinπ/3=(15+8根号3)/34