Kí hiệu nào sau đây dùng cho tập số hữu tỉ?

1.

a) \(A = 3:{\left( {\frac{{ - 3}}{2}} \right)^2} + \frac{1}{9}\,\,.\,\sqrt {36} + 0,75\)\( = 3:\frac{9}{4} + \frac{1}{9}.6 + 0,75\)

\( = 3\,\,.\,\,\frac{4}{9} + \frac{2}{3} + 0,75\)\(A = \frac{4}{3} + \frac{2}{3} + 0,75\)\( = 2 + 0,75\)\( = 2,75\).

b) \(B = \left( {8 - \frac{2}{3} + \frac{1}{2}} \right) - \left( {5 - \frac{7}{3} - \frac{3}{2}} \right) - \left( {\frac{5}{3} + \frac{5}{2} + 4} \right)\).

\[ = 8 - \frac{2}{3} + \frac{1}{2} - 5 + \frac{7}{3} + \frac{3}{2} - \frac{5}{3} - \frac{5}{2} - 4\]

\[ = \left( {8 - 5 - 4} \right) + \left( { - \frac{2}{3} + \frac{7}{3} - \frac{5}{3}} \right) + \left( {\frac{1}{2} + \frac{3}{2} - \frac{5}{2}} \right)\]

\[ = - 1 + 0 + \frac{{ - 1}}{2}\]\[ = \frac{{ - 3}}{2}\].

2.

a) \(\frac{3}{4} - \left( {x + \frac{1}{2}} \right) = \frac{1}{4}\)

\(x + \frac{1}{2} = \frac{3}{4} - \frac{1}{4}\)

\(x + \frac{1}{2} = \frac{1}{2}\)

\(x = \frac{1}{2} - \frac{1}{2}\)

\(x = 0\)

Vậy \(x = 0\).

b) \({\left( {x - \frac{2}{3}} \right)^2} + \frac{{16}}{{25}} = 1\)

\[{\left( {x - \frac{2}{3}} \right)^2} = 1 - \frac{{16}}{{25}}\]

\[{\left( {x - \frac{2}{3}} \right)^2} = \frac{9}{{25}}\]

\[{\left( {x - \frac{2}{3}} \right)^2} = {\left( {\frac{3}{5}} \right)^2} = {\left( {\frac{{ - 3}}{5}} \right)^2}\]

TH1: \(x - \frac{2}{3} = \frac{3}{5}\)

\(x = \frac{3}{5} + \frac{2}{3}\)

\(x = \frac{{19}}{{15}}\)

TH2: \(x - \frac{2}{3} = \frac{{ - 3}}{5}\)

\(x = \frac{{ - 3}}{5} + \frac{2}{3}\)

\(x = \frac{1}{{15}}\)

Vậy \[x \in \left\{ {\frac{{19}}{{15}};\,\,\frac{1}{{15}}} \right\}\].

Mỗi chiếc ti vi lãi số tiền là:

\(6{\rm{ }}400{\rm{ }}000\,\,.\,\,20\% = 1{\rm{ }}280{\rm{ }}000\) (đồng)

Nếu bán hết 20 chiếc ti vi thì số tiền lãi là:

\(20\,\,.\,\,1{\rm{ 280 000 = 25 600 000}}\) (đồng).

Vậy nếu bán hết 20 chiếc ti vi thì số tiền lãi là 25 600 000 đồng.