如图所示，有一个圆和一条弦 AB。B 在 A 的正东方。

1. Construct the required angle at B and locate C on the circumference

At $B$, place the protractor with its baseline along $BA$. Mark $110^\circ$ on the side above $AB$, then draw the ray from $B$ through this mark. Label the point where this ray meets the circle as $C$.

Thus $\angle ABC = 110^\circ$.

1. Construct the locus of points equidistant from $A$ and $B$

Draw the perpendicular bisector of $AB$. Since $D$ is equidistant from $A$ and $B$, $D$ must lie on this perpendicular bisector.

2. Locate the point on this locus that gives $\angle ADC=70^\circ$ and is above $AB$

Using the constructed point $C$, find the point on the perpendicular bisector of $AB$ such that the angle between $DA$ and $DC$ is $70^\circ$. Select the intersection above $AB$ and label it $D$.

1. Locate the centre using perpendicular bisectors of chords

Draw the perpendicular bisector of chord $AB$ and the perpendicular bisector of another chord, e.g. $AC$. Their point of intersection is the centre of the circle. Label this point $O$.