((派/2)

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((派/2)((派/2)((派/2)sin2A=sin[(A+B)+(A-B)]=sin(A+B)cos(A-B)+cos(A+B)sin(A-B)由于pi∵((派/2)∴A-B>0((派)∴sin(

((派/2)
((派/2)

((派/2)
sin2A=sin[(A+B)+(A-B)]
=sin(A+B)cos(A-B)+cos(A+B)sin(A-B)
由于pi

∵ ((派/2) ∴A-B>0 ((派) ∴sin(A-B)=5/13
cos(A+B)=-(4/5)
∴sin2A=sin(A+B+ A-B)= sin(A+B)cos(A-B)+ cos(A+B)sin(A-B)
=-(3/5)* 12/13-(4/5)*5/13
=-56/65