cos(a+π/4)=3/5cosacosπ/4-sinasin(π/4)=3/5cosa-sina=3√2/5cos²a-2sinacosa+sin²a=18/251-sin2a=18/25sin2a=7/25π/2≤a≤3π/23π/4≤a+π/4≤7π/4因为cos(a+π/4)=3/5>0所以3π/2≤a+π/4≤7π/45...