cos(π/4+x)=cosπ/4cosx-sinπ/4sinx=(1/√2)(cosx-sinx)=3/5,cosx-sinx=3√2/5,两边平方得,cos²‎x+sin²‎x-2cos‎xsinx=18/25,1-sin2x=18/25,sin2x=7/18打字不易,