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Gọi bán kính hình tròn là r thì diện tích hình tròn là r x r x 3,14
Nếu tăng bán kính lên 20% thì bán kính mới là:r + r x 20%=r x (1+20%)
=r x 120%
Diện tích hình tròn mới là:
r x 120% x r x120% x 3,14= r x r x3,14 x(120/100 x 120/100)
=r x r x 3,14 x 144%
Diện tích hình tròn tăng lên số phần trăm là:
r x r x3,14 x 144% - r x r x3,14
=44%
Vậy nếu tăng bán kính hình tròn lên 20% thì diện tích hình tròn sẽ tăng 44%
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Chu vi cu = r x 2 x 3,14
Chu vi moi = r x 2 x 2 x 3,14 = r x 4 x 3,14
( 4 x 3,14 ) : ( 2 x 3,14 ) = 2
Vay chu vi tang gap 2 lan
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
Chu vi hình tròn ban đầu là: rx2x3,14
Chu vi hình tròn mới là: (rx2)x2x3,14 = (rx2x2,14)x2
Như vậy chu vi hình tròn mới tăng lên 200%
ĐS: 200%
ADCT: P=r.2.3,14 tăng thì P=r.2.2.3,14. Vậy chu vi tăng gấp đôi và tăng 100%
![](https://rs.olm.vn/images/avt/0.png?1311)
Sai, phải là 200% vì:
Chu vi hình tròn ban đầu là: r * 2 * 3,14
khi tăng bán kính hình tròn lên 2 lần thì chu vi hình tròn đó là: r * 2 * 2 * 3,14 = 2 * 100= 200%
vậy chu vi hình tròn nó tăng lên 200%
Chu vi hình tròn tăng lên:
12 x 3,14 = tự tính
Ta có công thức tính chu vi hình tròn là: r x 2 x 3,14 = V
=> Khi bán kính gấp lên 12% thì chu vi gấp lên :
12% x 2 = 24%
Đáp số: 24%