π/4<α<3π/4π/2<α+π/4<πcos(α+π/4)= - √[1-(4/5)^2] = - 3/5cosα=cos[(α+π/4)-π/4]=cos(α+π/4)cos(π/4)+sin(α+π/4)sin(π/4) =(4/5)(√2/2)+(3/5)(√2/2) =7√2/10
有点难
不会
