72 398 : 32 = 2 262 dư 14 

\(-0,625=-\dfrac{625}{1000}=-\dfrac{5}{8}\)

a) Phân số đó là: \(\dfrac{5}{-8};\dfrac{20}{-32};\dfrac{-10}{16};\dfrac{-25}{40}\)

b)  

A: \(Cu_xO_y\)

% Cu = 100% - 20 % = 80%

\(x:y=\dfrac{\%Cu}{M_{Cu}}:\dfrac{\%O}{M_O}=\dfrac{80}{64}:\dfrac{20}{16}=1,25:1,25=1:1\)

=> A:\(Cu_1O_1=CuO\)

Số tiền Mai dùng mua quà tặng mẹ:

   648 000 : 2 = 324 000 (đồng)

Giá tiền cái mũ:

    (324 000 - 60 000) : 2 = 132 000 (đồng)

Giá tiền cái khăn:

    132 000 + 60 000 = 192 000 (đồng)

Số đối của -0,75 là 0,75

Số nguyên tố: 5; 11; 17

Hợp số: 6; 10; 16

\(1^2=1,2^2=4,3^2=9,4^2=16,5^2=25\\ 6^2=36,7^2=49,8^2=64,9^2=81,10^2=100\\ 11^2=121,12^2=144,13^2=169,14^2=196,15^2=225\\ 16^2=256,17^2=289,18^2=324,19^2=361,20^2=400\\ 21^2=441,22^2=484,23^2=529,24^2=576,25^2=625\\ 26^2=676,27^2=729,28^2=784,29^2=841,30^2=900\)

\(A=\left(1+3+3^2+3^3\right)+...+\left(3^8+3^9+3^{10}+3^{11}\right)\\ =40+...+3^8\left(1+3+3^2+3^3\right)\\ =40+...+3^8.40\\ =40\left(1+...+3^8\right)⋮5,8\)

\(A=\left(1+3^2\right)+\left(3+3^3\right)+...+\left(3^8+3^{10}\right)+\left(3^9+3^{11}\right)\\ =10+3\left(1+3^2\right)+...+3^8\left(1+3^2\right)+3^9\left(1+3^2\right)\\ =10+3.10+...+3^8.10+3^9.10\\ =10\left(1+3+...+3^8+3^9\right)⋮5\)

\(B=\left(1+5+5^2\right)+\left(5^3+5^4+5^5\right)+\left(5^6+5^7+5^8\right)\\ =31+5^3\left(1+5+5^2\right)+5^6\left(1+5+5^2\right)\\ =31+5^3.31+5^6.31\\ =31.\left(1+5^3+5^6\right)⋮31\)