sin(a-b)sin(b-r)-cos(a-b)cos(r-b)
=-sin(a-b)sin(r-b)-cos(a-b)cos(r-b)
=-[cos(a-b)cos(r-b)+sin(a-b)sin(r-b)]
=-cos[(a-b)-(r-b)]
=-cos(a-b-r+b)
=-cos(a-r)