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,
已知sin（x-π/4）=3/5,∈（π/4,π/2）,求cos2x的值?
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