Why the surface area of cone = pi*r*l +pi*r^2
i know l = sqrt ( r^2+h^2)
but wanna ask how to get pi*r*l this concept?
Surface area of the cone
2007-05-28 6:27 am
回答 (2)
2007-05-28 6:40 am
參考: My Maths knowledge
2007-05-28 6:46 am
做曲面既面積嘛
將佢翻開係咪一個 sector呀,
sector 個半徑其實係個 cone 個斜長 l
sector 個曲長其實係個 cone 底既周界 2 pi r
sector 面積 = 1/2 R^2 夾角
= 1/2 R 曲長 (因為 曲長係 R 夾角)
= 1/2 (l) (2 pi r)
= pi * r * l
就係咁,自己睇下明唔明
將佢翻開係咪一個 sector呀,
sector 個半徑其實係個 cone 個斜長 l
sector 個曲長其實係個 cone 底既周界 2 pi r
sector 面積 = 1/2 R^2 夾角
= 1/2 R 曲長 (因為 曲長係 R 夾角)
= 1/2 (l) (2 pi r)
= pi * r * l
就係咁,自己睇下明唔明
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