cos(11π/8)=cos(π+3π/8)=-cos(3π/8)
=-sin(π/2-3π/8)=-sin(π/8)
sin(10π/9)=sin(π+π/9)=-sin(π/9)
由于π/8和π/9都在第一象限,且π/8>π/9,所以：
sin(π/8)>sin(π/9)
或：
-sin(π/9)>-sin(π/8)
所以：sin(10π/9)>cos(11π/8)