The picture I have is a right angle ABC. The line BS lies in between lines AB and BC. The point D lies along BS so that when joined to point C DC is perpendicular to BS.
<ABS=90-SBC (because <ABC is a right angle and <ABC=90=ABS+SBC),
<DCB=90-SBC (because BDC is a right-angled triangle where D is the right angle so the other two angles must add up to 90 degrees to make up 180 degrees for the whole triangle),
therefore, <ABS=DCB.