Tính nhanh:

a) (11 + 13 + 15 + …. + 19) × (6 × 8 – 48)

b) (72 – 8 × 9) : (20 + 21 + 22 + 23 + 24 + 25)

a) \(6 + 6 + 6 + ... + 6 - 66\)( có \(11\) số \(6\))

\( = 6 \times 11 - 66\)

\( = 66 - 66\)

\( = 0\)

b) \(7 + 7 + 7 + ... + 7 + 223\) ( có \(11\) số \(7\))

\( = 7 \times 11 + 223\)

\( = 77 + 223\)

\( = 300\)

c) \(\underbrace {50 + 50 + 50 + ... + 50 - 50}_{20\,\,so\'a \,\,50} - 18 \times 50\)

\( = 50 \times 19 - 50 - 18 \times 50\)

\( = 50 \times \left( {19 - 1 - 18} \right)\)

\( = 50 \times 0\)

\( = 0\)

d) \(\underbrace {9 + 9 + 9 + ... + 9 - 9}_{100\,\,so\'a \,\,9} - 8 \times 9 + 18 \times 5\)\( = 9 \times 99 - 9 - 8 \times 9 + 18 \times 5\)\( = 9 \times \left( {99 - 1 - 8} \right) + 18 \times 5\)

\( = 9 \times 90 + 9 \times 2 \times 5\)

\( = 9 \times 90 + 9 \times 10\)

\( = 9 \times \left( {90 + 10} \right)\)

\( = 9 \times 100\)

\( = 900\)