证明cos5π/12=cos(π/2-5π/12)=sinπ/12∴cos²5π/12+cos²π/12=sin²π/12+cos²π/12=1（cos5π/12）（cosπ/12）=sinπ/12*cosπ/12=1/2*2sinπ/12*cosπ/12.正弦二倍角公式=1/2*sinπ/6=1/2...