sin(a+π/4)=3/5a∈(-π/4,π/4)得 cosa>0sin(a+π/4)=(√2/2)(sina+cosa)=3/5得 sina+cosa=3√2/5平方得 1+2sinacosa=18/25得 2sinacosa=-7/25所以 (sina-cosa)²=1+7/25=32/25因为 a∈(-π/4,π/4)所以 cosa>s...