tính nhanh nếu có thể): a) 349 + 54 + 651 + 1 946; b) 80-(3.(5^2)-4.(2^3));

a) \[349 + 54 + 651 + 1\,\,946\]

\[ = \left( {349 + 651} \right) + \left( {54 + 1946} \right)\]

\[ = 1000 + 2000 = 3000\].

b)\[\;80--\left( {3 \cdot {5^2}--4 \cdot {2^3}} \right)\]

=\[\;80--(3 \cdot 25--4 \cdot 8)\]

=\[\;80--(75--32)\]

=\[\;80--43 = 37\].

c) \[39 \cdot 65 + 39 \cdot 36 - 39\]

=\[39 \cdot \left( {65 + 36 - 1} \right)\]

=\[39 \cdot 100 = 3900\].