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(20 1 1? Xiaohong needs to make a conical birthday hat with a bottom radius of 9 cm and a height of 12 cm with fan-shaped paper towels.
∫ The bottom radius is 9 cm, and the height is 12 cm.
∴ bus cone length = 122+92= 15cm,
The radius of the bottom is 9 cm,
∴ Bottom circumference = 18πcm,
∴nπ× 15 180= 18π，
The solution is n=2 16,
The central angle of the sector paperboard is 2 16.