《2025年天天练口算六年级数学上册人教版》

答案： 1. 当$d = 40cm$时：
圆的周长公式$C=\pi d$，$\pi$取$3.14$，则$C = 3.14×40=125.6cm$；
半径$r=\frac{d}{2}=\frac{40}{2}=20cm$，圆的面积公式$S = \pi r^{2}$，则$S=3.14×20^{2}=3.14×400 = 1256cm^{2}$。
2. 当$d = 6cm$时：
$C=\pi d$，$\pi$取$3.14$，则$C = 3.14×6 = 18.84cm$；
$r=\frac{d}{2}=\frac{6}{2}=3cm$，$S=\pi r^{2}$，则$S = 3.14×3^{2}=3.14×9 = 28.26cm^{2}$。
3. 当$r = 8cm$时：
$C = 2\pi r$，$\pi$取$3.14$，则$C=2×3.14×8=50.24cm$；
$S=\pi r^{2}$，则$S = 3.14×8^{2}=3.14×64 = 200.96cm^{2}$。
4. 当$r = 4cm$时：
$C = 2\pi r$，$\pi$取$3.14$，则$C=2×3.14×4 = 25.12cm$；
$S=\pi r^{2}$，则$S = 3.14×4^{2}=3.14×16 = 50.24cm^{2}$。
5. 当$C = 94.2cm$时：
由$C = 2\pi r$，可得$r=\frac{C}{2\pi}$，$\pi$取$3.14$，则$r=\frac{94.2}{2×3.14}=\frac{94.2}{6.28}=15cm$；
$S=\pi r^{2}$，则$S = 3.14×15^{2}=3.14×225 = 706.5cm^{2}$。
故答案依次为：$125.6cm$，$1256cm^{2}$；$18.84cm$，$28.26cm^{2}$；$50.24cm$，$200.96cm^{2}$；$25.12cm$，$50.24cm^{2}$；$15cm$，$706.5cm^{2}$。

$\frac{1}{4}:\frac{1}{6}=$3:2
$\frac{1}{20}:\frac{1}{30}=$3:2
$\frac{7}{18}:\frac{14}{24}=$2:3
$8:1.2=$20:3
$5:15=$1:3
$0.2:0.06=$10:3
$45:6=$15:2
$25:30=$5:6
$0.45:3=$3:20
$12:4=$3:1
$0.8:1.2=$2:3
$\frac{1}{8}:\frac{5}{2}=$1:20
$\frac{2}{3}:\frac{4}{3}=$1:2
$\frac{3}{8}:\frac{9}{2}=$1:12
$\frac{5}{6}:\frac{5}{3}=$1:2

答案： 解析：题目给出了多组比值，需要分别求出它们的简化结果。这涉及到比的基本性质和化简。
答案：
1. $\frac{1}{4}:\frac{1}{6} = \frac{1}{4} ÷ \frac{1}{6} = \frac{1}{4} × 6 = \frac{6}{4} = \frac{3}{2}=3:2$
2. $\frac{1}{20}:\frac{1}{30} = \frac{1}{20} ÷ \frac{1}{30} = \frac{1}{20} × 30 = \frac{30}{20} = \frac{3}{2}=3:2$
3. $\frac{7}{18}:\frac{14}{24} = \frac{7}{18} ÷ \frac{14}{24} = \frac{7}{18} × \frac{24}{14} = \frac{7 × 24}{18 × 14} = \frac{2}{3}=2:3$
4. $8:1.2 = 8 ÷ 1.2 = \frac{8}{1.2} = \frac{80}{12} = \frac{20}{3}=20:3$
5. $5:15 = 5 ÷ 15 = \frac{5}{15} = \frac{1}{3}=1:3$
6. $0.2:0.06 = 0.2 ÷ 0.06 = \frac{0.2}{0.06} = \frac{20}{6} = \frac{10}{3}=10:3$
7. $45:6 = 45 ÷ 6 = \frac{45}{6} = \frac{15}{2}=15:2$
8. $25:30 = 25 ÷ 30 = \frac{25}{30} = \frac{5}{6}=5:6$
9. $0.45:3 = 0.45 ÷ 3 = \frac{0.45}{3} = \frac{45}{300} = \frac{3}{20}=3:20$
10. $12:4 = 12 ÷ 4 = 3=3:1$
11. $0.8:1.2 = 0.8 ÷ 1.2 = \frac{0.8}{1.2} = \frac{8}{12} = \frac{2}{3}=2:3$
12. $\frac{1}{8}:\frac{5}{2} = \frac{1}{8} ÷ \frac{5}{2} = \frac{1}{8} × \frac{2}{5} = \frac{1 × 2}{8 × 5} = \frac{1}{20}=1:20$
13. $\frac{2}{3}:\frac{4}{3} = \frac{2}{3} ÷ \frac{4}{3} = \frac{2}{3} × \frac{3}{4} = \frac{1}{2}=1:2$
14. $\frac{3}{8}:\frac{9}{2} = \frac{3}{8} ÷ \frac{9}{2} = \frac{3}{8} × \frac{2}{9} = \frac{3 × 2}{8 × 9} = \frac{1}{12}=1:12$
15. $\frac{5}{6}:\frac{5}{3} = \frac{5}{6} ÷ \frac{5}{3} = \frac{5}{6} × \frac{3}{5} = \frac{1}{2}=1:2$