Bài 1 trang 88 sgk Toán 7 tập 2, Thực hiện các phép tính...

Thực hiện các phép tính:

a) $9,6.2{1 \over 2} - \left( {2.125 - 1{5 \over {12}}} \right):{1 \over 4}$

b) ${5 \over {18}} - 1,456:{7 \over {25}} + 4,5.{4 \over 5}$;

c) $\left( {{1 \over 2} + 0,8 - 1{1 \over 3}} \right).\left( {2,3 + 4{7 \over {25}} - 1,28} \right)$

d) $\left( { - 5} \right).12:\left[ {\left( { - {1 \over 4}} \right) + {1 \over 2}:\left( { - 2} \right)} \right] + 1{1 \over 3}$.

Hướng dẫn làm bài:

a) $9,6.2{1 \over 2} - \left( {2.125 - 1{5 \over {12}}} \right):{1 \over 4}$

$= 9,6.{5 \over 2} - \left( {250 - {{17} \over {12}}} \right) \times 4$

$= 4,8.5 - \left( {1000 - {{17} \over 3}} \right)$

$= 24 - 1000 + {{17} \over 3}$

$=  - 976 + {{17} \over 3}$

$=  - 970{1 \over 3}$

b) ${5 \over {18}} - 1,456:{7 \over {25}} + 4,5.{4 \over 5}$;

$= {5 \over {18}} - 1,456 \times {{25} \over 7} + {9 \over 2}.{4 \over 5}$

$= {5 \over {18}} - 0,208 \times 25 + {{18} \over 5}$

$= {5 \over {18}} - 5,2 + {{18} \over 5}$

$= {{25 - 468 + 324} \over {90}}$

$= {{ - 119} \over {90}}$

c) $\left( {{1 \over 2} + 0,8 - 1{1 \over 3}} \right).\left( {2,3 + 4{7 \over {25}} - 1,28} \right)$

$= \left( {{1 \over 2} + {4 \over 5} - {4 \over 3}} \right).\left( {{{23} \over {10}} + {{107} \over {25}} - {{32} \over {25}}} \right)$

$= \left( {{{15 + 24 - 40} \over {30}}} \right).\left( {{{23} \over {10}} + {{107} \over {25}} - {{32} \over {25}}} \right)$

$= \left( {{{15 + 24 - 40} \over {30}}} \right).\left( {{{115 + 214 - 64} \over {50}}} \right)$

$= {{ - 1} \over {30}}.{{265} \over {50}}$

$= {{ - 53} \over {300}}$

d) $\left( { - 5} \right).12:\left[ {\left( { - {1 \over 4}} \right) + {1 \over 2}:\left( { - 2} \right)} \right] + 1{1 \over 3}$

$=  - 60:\left[ {{1 \over 4} + {1 \over 2} \times \left( {{{ - 1} \over 2}} \right)} \right] + 1.{1 \over 3}$

$=  - 60:\left[ { - {1 \over 4} - {1 \over 4}} \right] + 1{1 \over 3}$

$=  - 60:\left( {{1 \over 2}} \right) + 1{1 \over 3}$

$= 120 + 1{1 \over 3}$

$= 121{1 \over 3}$