(派/2)

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(派/2)(派/2)(派/2)派/20cos(A-B)=12/13sin(A-B)=5/13sin(A+B)=-(3/5)派cos(A+B)=-(4/5)sin2A=sin(A-B+A+B)=s(A-

(派/2)
(派/2)

(派/2)
派/20cos(A-B)=12/13
sin(A-B)=5/13
sin(A+B)=-(3/5)
cos(A+B)=-(4/5)
sin2A=sin(A-B+A+B)=s(A-B)c(A+B)+c(A-B)s(A+B)=-(5/13)x(4/5)-(12/13)x(3/5)=-(56/65)

sin2A=sin(A+B +A-B)=sin(A+B)cos(A-B)+cos(A+B)sin(A-B)
A+B在派到2/3派间,因而cos(A+B)小于零。A-B在零到四分之派间,因而sin(A-B)大于零。