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What is the bus length formula? One of LR.
Mathematically, the line from the apex of the cone to any point on the circumference of the bottom of the cone is called the generatrix of the cone;

Cut along any generatrix of the cone and expand it into a plane figure, that is, a sector;

The radiu of that expanded sector is the generatrix of the cone,

The arc length of the expanded sector is the circumference of the cone bottom;

Through expansion, the side area of three-dimensional graphics is converted into the area of plane graphics.

Solution: Let the generatrix length of the cone be L and the radius of the bottom surface of the cone be R,

Then the fan-shaped radius is L, and the arc length is the circumference of the cone bottom (2πR).

We already know that the formula of sector area is: S = (1/2)× sector radius× sector arc length.

= ( 1/2)× L × (2πR)

= π R L

That is, the lateral area of the cone is π times the product of the radius of the cone bottom surface and the length of the cone generatrix.