Geometry: using only compasses and straightedge
Divde an angle in half. (Bisect an angle.)
Draw two lines taht meet at B. Use your compasses centred at B to draw arcs of a circle ctting the two kines at the red dots. These red dots are now the same distance from B.
Draw two circles the same size, keeping the compasses set at the same width, with centres at the red dots.
A line through B and where the two circles meet divides the angle in two.