作业帮cos(a-B/2)=-5/13,sin(a/2-B)=4/5,且0
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cos(a-B/2)=-5/13,sin(a/2-B)=4/5,且0 cos(a-B/2)=-5/13,sin(a/2-B)=4/5,且0
cos(a-B/2)=-5/13,sin(a/2-B)=4/5,且0
sin^2a+cos^2a=1;
sin^2(a-b/2)=144/169,0＜a-b/2＜180o；
sin(a-b/2)=12/13;
cos^2(a/2-b)=9/25; 0 ＜ a/2-b＜90o;
cos(a/2-b)=3/5;
又（a+b)/2=(a-b/2)+(a/2-b);
sin(a+b)=2sin((a+b)/2)cos((a+b)/2)
=2sin[(a-b/2)+(a/2-b)]cos[(a-b/2)+(a/2-b)]
=2[sin(a-b/2)cos(a/2-b)+cos(a-b/2)sin(a/2-b)][cos(a-b/2)cos(a/2-b)-sin(a-b/2)sin(a/2-b)]
=-32*63/65*65
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