In the diagram,
α\alphaα = 180∘^{\circ}∘ - 177∘^{\circ}∘ (angles on a straight line)
α\alphaα = 63∘^{\circ}∘
t = 2 x 63∘^{\circ}∘ (angle at centre = 2 x angle at circum)
= 126∘^{\circ}∘
