ABC <A+<C=180 -LB.
AOC <OAC+<OCA=(<A+<C)/2=(180-<B)/2=90-<B/2
<AOC=180-(<AOC+<<OCA)=180-(90-<<B/2)=90+<B/2
ABC <A+<C=180 -LB.
AOC <OAC+<OCA=(<A+<C)/2=(180-<B)/2=90-<B/2
<AOC=180-(<AOC+<<OCA)=180-(90-<<B/2)=90+<B/2