1、甲店买12瓶 = 9/5×12×0.8 = 17.28元
乙店买12瓶 = 9/5×10 = 18元, 所以在甲店买便宜.
2、直径d = 4cm×2 = 8cm.
3、在这张纸板上能剪出最大的圆的直径是2分米,所以半径是1分米.
4、①r=30cm,周长C = 2πr = 2×3.14×30 = 188.4cm；
②d=10cm,周长C = πd = 3.14×10 = 31.4cm；
③r=4.5cm,周长C = 2πr = 2×3.14×4.5 = 28.26cm；
④d=2.5cm,周长C = πd = 3.14×2.5 = 7.85.
5、占地面积S = 80×40+3.14×20² = 3200+1256 = 4456cm².
6、树的周长C = 376.8÷5 = 75.36cm,则r=75.36÷2π = 12cm,
所以横截面积S = πr² = 452.16cm².
7、截去的半圆r=2dm,则面积S = πr²÷2 = 3.14×4÷2 = 6.28cm².
8、草坪面积S = (20²-15²)×3.14 = 549.5cm².
9、树干半径r=6.28÷3.14÷2=1米,所以横截面积S = 2×1×3.14 = 6.28平方米.
10、面积S = (11²-7²)×3.14 = 226.08平方厘米.