In the derivation of the cylinder formula, the square body surface area is 20 cm larger than the cylinder, what is the cylinder side area? In deriving the cylinder formula, the square body surface area is 20 cm larger than the cylinder, what is the cylinder side area?

In the derivation of the cylinder formula, the square body surface area is 20 cm larger than the cylinder, what is the cylinder side area? In deriving the cylinder formula, the square body surface area is 20 cm larger than the cylinder, what is the cylinder side area?

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The Derivation Process of Surface Area Formula of Sphere How did the surface area formula of the ball come out? Why did I push it so many times? *R square, the principle is to cut the hemisphere into one hemisphere, and then cut the hemisphere into countless small triangles. The sum of the bottom of the small triangles is the circumference (2πR), the height is a quarter of the circumference (1/2πR), the surface area of the circle is (2πR*1/2πR)/2*2=π square*R square, is my principle wrong or the height of the small triangles is not a quarter of the circumference (1/2πR)? If my principle is wrong, please tell me the principle. If the height of the small triangle is wrong, please tell me the height of the small triangle and prove it. Thank you

Let the circle y=√(R^2-x^2) rotate around the x axis to obtain the sphere x^2+y^2+z^2≤R^2. Find the surface area of the sphere.
With x as the integral variable, the integral limit is [-R, R].
In [-R, R], any subinterval [x, x x], the area of the upper part of the sphere obtained by this arc around the x-axis is approximately 2 y×ds, ds is the arc length.
Therefore, the surface area of the ball S=∫2 y (1+y'^2) dx, and then S=4πR