sin(x-π/4) = 3/5sinx·cosπ -cosx·sinπ = 3/5√2/2 sinx-√2/2 cosx = 3/5由上式得：sinx-cosx = 3/5 * √2等式两端同时取平方,得：(cosx)^2 + (sinx)^2 - 2sinx cosx = 9/25*21-sin2x = 18/25所以,sin2x=7/25...