sin(3π/4+b)=sin(b-π/4),sin(a+b)=sin(π/4+a+b-π/4)=sin(π/4+a)cos(b-π/4)+cos(π/4+a)sin(b-π/4)
sin(π/4+a)=根号下(1-9/25),a+π/4在第2象限,取正得4/5
cos(b-π/4)=根号下(1-25/169),b-π/4在第4象限,取正得12/13
sin(a+b)=sin(π/4+a+b-π/4)=sin(π/4+a)cos(b-π/4)+cos(π/4+a)sin(b-π/4)=
48/65-15/65=33/65