由sin a=1/3,得：cos²a=1-sin²a=1-1/9=8/9,因为a∈(π/2，π)，所以cos a=-2√2 /3,所以cos(π/4-a)=cosπ/4cosa+ sinπ/4sina=√2/2×(-2√2/3)+√2/2×(1/3)=(-4+√2)/6
