(a+2b,a+b）屬于（0~2π）
sin(a+2b)=4/5,cos(a+2b)=±根號(1-sin^2(a+2b))=±3/5
cos(a+b)=12/13 ,sin(a+b)=±根號(1-cos^2(a+b))=±5/13
sin(a+2b)cos(a+b)=4/5 * 12/13 = 48/65
cos(a+2b)sin(a+b)=(±3/5) * (±5/13)=15/65,或-15/65
sinb=sin{(a+2b)-(a+b)}
=sin(a+2b)cos(a+b)-cos(a+2b)sin(a+b)
=48/65-15/65=33/65
或：
=48/65-（-15/65）=63/65