Khẳng định nào sau đây là sai?

a) \( - 2,1 - \left( { - 0,6} \right) + 0,4\)

\( =  - 2,1 + 0,6 + 0,4\)

\( =  - 2,1 + \left( {0,6 + 0,4} \right)\)

\( =  - 2,1 + 1 =  - 1,1\).

c) \(\frac{6}{5}.1\frac{1}{3} + \left( {\frac{1}{4} - \frac{5}{6}} \right):4\frac{2}{3}\)

\( = \frac{6}{5}.\frac{4}{3} + \left( {\frac{3}{{12}} - \frac{{10}}{{12}}} \right):\frac{{14}}{3}\)

\( = \frac{8}{5} + \frac{{ - 7}}{{12}}.\frac{3}{{14}}\)

\( = \frac{8}{5} + \frac{{ - 1}}{8}\)

\( = \frac{{64}}{{40}} - \frac{5}{{40}} = \frac{{59}}{{40}}\).

b) \(\frac{1}{4} + \frac{2}{5}\,\,.\,\,\frac{{ - 3}}{2}\)

\( = \frac{1}{4} + \frac{{ - 3}}{5}\)

\( = \frac{5}{{20}} - \frac{{12}}{{20}}\)

\( =  - \frac{7}{{20}}\);

d) \(\frac{2}{{11}}.\frac{{ - 5}}{4} + \frac{{ - 9}}{{11}}.\frac{5}{4} + 1\frac{3}{4}\)

\( = \frac{2}{{11}}.\frac{{ - 5}}{4} + \frac{9}{{11}}.\frac{{ - 5}}{4} + \frac{7}{4}\)

\( = \frac{{ - 5}}{4}.\left( {\frac{2}{{11}} + \frac{9}{{11}}} \right) + \frac{7}{4}\)

\( = \frac{{ - 5}}{4}.1 + \frac{7}{4}\)

\[ = \frac{5}{4} + \frac{7}{4}\]

\[ = \frac{{12}}{4} = 3\].

a) \(0,75 - x =  - 0,6\)

\(x = 0,75 - \left( { - 0,6} \right)\)

\(x = 0,75 + 0,6\)

\(x = 1,35\)

Vậy \(x = 1,35\).

b) \[x + \frac{7}{2} = \frac{{15}}{4}\]

\[x = \frac{{15}}{4} - \frac{7}{2}\]

\[x = \frac{{15}}{4} - \frac{{14}}{4}\]

\[x = \frac{1}{4}\]

Vậy \[x = \frac{1}{4}\].

c) \(\left( {{x^2} + 1} \right)\left( {3x - 4} \right) = 0\)

Trường hợp 1:

\({x^2} + 1 = 0\)

Vì \({x^2} \ge 0\) nên \({x^2} + 1 > 0\).

Do đó \({x^2} + 1 \ne 0\).

Vậy \(x = \frac{4}{3}\).

Trường hợp 2:

\(3x - 4 = 0\)

\(3x = 4\)

\(x = \frac{4}{3}\)