1. 1/2× sector arc length× sector radius (sector radius is the generatrix length of the cone).

2.nπR？ /360 (where n represents the degree of the central angle and r is the radius of the sector)

The side development diagram of the cone is a sector, and the circumference of the side part is equal to the arc length of 2R+ sector.

It is not told here that the arc length of the sector can also be calculated by the circumference of the cone bottom circle (r is the radius of the cone bottom circle).

That is, the cone lateral area can also be =πLR.