Thực hiện phép tính (tính hợp lý nếu có thể): a) 4.2^2-21:3; b) 86.19 + 19.15 - 19;

a) \[4 \cdot {2^2}--21:3\]

\[ = 4 \cdot 4--21:3\]

\[ = 16--7 = 9\].

b) \[86 \cdot 19 + 19 \cdot 15 - 19\]

\[ = 19 \cdot \left( {86 + 15 - 1} \right)\]

\[ = 19 \cdot 100 = 1900\].

c) \[{7^{20}}:\left( {{7^{16}} \cdot 6 + {7^{16}} \cdot 43} \right)\]

\[ = {7^{20}}:\left[ {{7^{16}} \cdot \left( {6 + 43} \right)} \right]\]

\[ = {7^{20}}:{7^{16}}:49 = {7^4}:{7^2}\]

\[ = {7^2} = 49\].

d) \[720 - \left\{ {20.\left[ {\left( {140 - 15} \right):25 + {3^2}} \right]} \right\} + {2024^0}\]

\[ = 420 - \left\{ {20 \cdot \left[ {{\rm{125}}:25 + {\rm{9}}} \right]} \right\} + 1\]

\[ = 420 - \left\{ {20 \cdot \left[ {5 + {\rm{9}}} \right]} \right\} + 1\]

\[ = 420 - \left\{ {20 \cdot 14} \right\} + 1\]

\[ = 420 - 280 + 1 = 141\].