已知sin(a+π/4)sin(a-π/4)=1/3,若π/2
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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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