sin（π/6）=3/5,所以有x=π/6+π/4=5π/12
2x=5π/6
cos(2x)=cos(5π/6)=-cos(π-5π/6)=-cos(π/6)=-4/5
打字不易,哎，数学不好的伤不起啊....谢谢亲~