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# 4.3 ??? - PowerPoint PPT Presentation

4.3 圆面积. 期末倒计时. 把一个圆 12 等分. 对半分. 半个圆先拼接. 另外半个继续拼接. 最后一片处理一下就成. 对半开. （注： C=2πr ）半个圆周长 πr. 6. 1. 2. 3. 4. 5. r. 圆的面积近似可以看成长方形面积. S=πrxr=πr 2. 圆的半径扩大 3 倍，面积扩大几倍？ 假设原来的半径为 r , 那么 S=π r 2 扩大三倍后半径为 3r ，那么 S=πx3rx3r=9πr 2 ∴ 是原来的 9 倍。 那么直径扩大 3 倍呢？

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## PowerPoint Slideshow about '4.3 ???' - aaron-roberts

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Presentation Transcript

4.3圆面积

（注：C=2πr）半个圆周长πr

6

1

2

3

4

5

r

S=πrxr=πr2

∴是原来的9倍。

【结论】圆面积S与圆的半径r或者直径d扩大、缩小的倍数成平方关系。

S圆环=πR2– π r2 =πx(R2- r2 )

RESUME已知甲圆的半径长是乙圆的直径长，且他们的面积之和是