∵sin(α-π/4)=⅓，又sin²(α-π/4)+cos²(α-π/4)=1∴cos²(α-π/4)=1-sin²(α-π/4)=1-⅓²=8/9∵α为锐角∴cosα>0∴cosα=√(8/9)=2√2/3
