点P在第一象限,且x=sin(π/5),y=cos(π/5)
则：
tana=y/x=[cos(π/5)]/[sin(π/5)] 【cos(π/5)=sin(3π/10)、sin(π/5)=cos(3π/10)】
tana=[sin(3π/10)]/[cos(3π/10)]=tan(3π/10)
则：a=2kπ＋(3π/10)