a)\(\frac{5}{7} - \left( { - 3,9} \right) = \frac{5}{7} + 3,9 = \frac{5}{7} + \frac{{39}}{{10}} = \frac{{50}}{{70}} + \frac{{273}}{{70}} = \frac{{323}}{{70}}\);

b)\(\left( { - 3,25} \right) + 4\frac{3}{4} =  - \frac{{13}}{4} + \frac{{19}}{4} = \frac{6}{4} = \frac{3}{2}.\)

a) \(\left( { - 3} \right).7 =  - \left( {3.7} \right) =  - 21\)

b) \(\left( { - 8} \right).\left( { - 6} \right) = 8.6 = 48\)

c) \(\left( { + 12} \right).\left( { - 20} \right) =  - \left( {12.20} \right) =  - 240\)

d) \(24.\left( { + 50} \right) = 24.50 = 1200\)

a) \(\left( { - 3} \right).\left( { - 2} \right).\left( { - 5} \right).4\)\( = \left[ {\left( { - 3} \right).\left( { - 2} \right)} \right].\left( { - 5} \right).4\)\( = 6.\left( { - 5} \right).4 =  - 30.4 =  - 120\).

b) \(3.2.\left( { - 8} \right).\left( { - 5} \right)\)\( = 3.2.\left[ {\left( { - 8} \right).\left( { - 5} \right)} \right] = 6.40\)\( = 240\).

\(a,=\left(4x^2-25\right)-\left(6x^2+9x-4x-6\right)\\ =4x^2-25-6x^2-5x+6=-2x^2-5x-19\\ b,=4x^2-4x+1-4\left(x^2-4\right)\\ =4x^2-4x+1-4x^2+16\\ =-4x+17\)

a) \(\left( { - 5,24} \right):1,31 =  - \left( {5,24:1,31} \right)\)\( =  - \left( {524:131} \right) =  - 4\)

b) \(\left( { - 4,625} \right):\left( { - 1,25} \right) = 4,625:1,25\)\( = 4625:1250 = 3,7\)

a)\({\left( { - 2} \right)^2}.{\left( { - 2} \right)^3} = {\left( { - 2} \right)^{2 + 3}} = {\left( { - 2} \right)^5}\);

b)\({\left( { - 0,25} \right)^7}:{\left( { - 0,25} \right)^5} = {\left( { - 0,25} \right)^{7 - 5}} = {\left( { - 0,25} \right)^2} = {\left( {0,25} \right)^2}\);

c)\({\left( {\frac{3}{4}} \right)^4}.{\left( {\frac{3}{4}} \right)^3} = {\left( {\frac{3}{4}} \right)^{4 + 3}} = {\left( {\frac{3}{4}} \right)^7}.\)

a) Cách 1: \(73 - \left( {2 - 9} \right) = 73 - 2 + 9 = 80\);

Cách 2: \(73 - \left( {2 - 9} \right) = 73 -(-7)=73+7 = 80\)

b) Cách 1: \(\left( { - 45} \right) - \left( {27 - 8} \right) = \left( { - 45} \right) - 27+8 =-72+8=- 64\)

 Cách 2: \(\left( { - 45} \right) - \left( {27 - 8} \right) = \left( { - 45} \right) - 19 =  - 64\)

\(\begin{array}{l}a)\left[ {{{\left( {\dfrac{3}{7}} \right)}^4}.{{\left( {\dfrac{3}{7}} \right)}^5}} \right]:{\left( {\dfrac{3}{7}} \right)^7}\\ = {\left( {\dfrac{3}{7}} \right)^{4 + 5}}:{\left( {\dfrac{3}{7}} \right)^7}\\ = {\left( {\dfrac{3}{7}} \right)^9}:{\left( {\dfrac{3}{7}} \right)^7}\\ = {\left( {\dfrac{3}{7}} \right)^{9-7}}\\= {\left( {\dfrac{3}{7}} \right)^2}\\b)\left[ {{{\left( {\dfrac{7}{8}} \right)}^5}:{{\left( {\dfrac{7}{8}} \right)}^4}} \right].\left( {\dfrac{7}{8}} \right)\\ = {\left( {\dfrac{7}{8}} \right)^{5 - 4}}.\left( {\dfrac{7}{8}} \right)\\ = \left( {\dfrac{7}{8}} \right).\left( {\dfrac{7}{8}} \right)\\ = {\left( {\dfrac{7}{8}} \right)^2}\\c)\left[ {{{\left( {0,6} \right)}^3}.{{\left( {0,6} \right)}^8}} \right]:\left[ {{{\left( {0,6} \right)}^7}.{{\left( {0,6} \right)}^2}} \right]\\ = {\left( {0,6} \right)^{3 + 8}}:{\left( {0,6} \right)^{7 + 2}}\\ = {\left( {0,6} \right)^{11}}:{\left( {0,6} \right)^9}\\ = {\left( {0,6} \right)^{11-9}}\\={\left( {0,6} \right)^2}.\end{array}\)

a)

\(\begin{array}{l}0,6 + \left( {\frac{3}{{ - 4}}} \right) = \frac{6}{{10}} + \left( {\frac{{ - 3}}{4}} \right)\\ = \frac{{12}}{{20}} + \left( {\frac{{ - 15}}{{20}}} \right) = \frac{{12 + \left( { - 15} \right)}}{{20}}\\ = \frac{{ - 3}}{{20}}\end{array}\)                 

 b)

 \(\begin{array}{l}\left( { - 1\frac{1}{3}} \right) - \left( { - 0,8} \right) = \frac{{ - 4}}{3} + \frac{8}{{10}}\\ = \frac{{ - 4}}{3} + \frac{4}{5} = \frac{{ - 20}}{{15}} + \frac{{12}}{{15}} = \frac{{ - 8}}{{15}}.\end{array}\)

a: =0,6-0,75=-0,15

b: \(=-\dfrac{4}{3}+\dfrac{4}{5}=\dfrac{-20+12}{15}=-\dfrac{8}{15}\)