If you want to make a cone-shaped object, how can you find the fan-shaped object after the cone is expanded?

First measure the fan-shaped bus l with a ruler
Then measure the arc length of the sector, you can use a tape measure
Arc length = the circumference of the circle at the bottom of the cone, from which the radius of the circle can be calculated, and then the area of the ground circle can be calculated
The area of the sector is s = 1 / 2L * arc length

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