已知sin(a+π/4)sin(a-π/4)=1/3,若π/2

已知sin(a+π/4)sin(a-π/4)=1/3,若π/2
已知sin(a+π/4)sin(a-π/4)=1/3,若π/2

已知sin(a+π/4)sin(a-π/4)=1/3,若π/2
解sin(a+π/4)sin(a-π/4)
=1/2｛cos[（a+π/4)-(a-π/4)]-cos[（a+π/4)+(a-π/4)]}
=1/2[cosπ/2-cos（2a)]
=1/2[-cos（2a)]
=1/3
即cos（2a)=-2/3
由π/2

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