The cone development diagram consists of a sector (the side of the cone) and a circle (the bottom of the cone). (as shown in the figure) When drawing the development diagram of a specified cone, A (bus length) and D (base diameter) are generally known.

∫ The circumference of an arc AB=⊙O O o.

∴ arc AB=πd

∫ arc AB = 2π a (∠ 1/360)?

∴2πa(∠ 1/360 )=πd？

∴2a(∠ 1/360 )=d？

Bring a and d into 2a(≈ 1/360)= d to get the value of ∠ 1.

In this way, all the data needed to draw the expansion diagram are obtained. According to the data, we can draw the expansion diagram of the cone.