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設大圓的半徑為R,小圓的半徑為r,則:2R×R÷2-2r×r÷2=50, ; ; ; ; ; ; ; ; ; ; ; ; ;R2-r2=50(平方釐米);圓環的面積:πR2-πr2,=π×(R2-r2),=3.14×50,=157(平方釐米);答:圓環的面積是157平方釐米.
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