A as a vertex of a regular pentagon. B is a vertex of a regular octagon.
C and D are vertices of both polygons.

The perimeter of the octagon is 40cm.
Work out the length AB.

isn't it a pentagon? sense [3 (triangle) + 4 (rectangle)] - 2 (triangle and rectangle connected) = 5 (pentagon)

we know that

major arc is defined as the bigger arc connecting two points on the edge of a circle. measure of major arc is always more than ° or π radian

in this problem

the mayor arc is bac

find the ratio of the measure of the major arc to the measure of the minor arc

therefore

the answer is the option