已知sin（α +π/4）=7√2/10,cos2α =7/25,求cosα 的值sin(α +π/4)=(√2/2)(sinα+cosα)=7(√2)/10,故sinα+cosα=7/5.(1)cos2α=cos²α-sin²α=(cosα+sinα)(cosα-sinα)=(7/5)(cosα-sinα)=7/25...