用a代替
cos(π/6-a)=1/3
cos[π-(π/6-a)]=-1/3
cos(5π/6+a)=-1/3
2kπ-π/2则2kπ+π/3<5π/6+a<2kπ+5π/6
所以sin(5π/6+a)>0
sin²(5π/6+a)+cos²(5π/6+a)=1
所以sin(5π/6+a)=2√2/3