Thực hiện phép tính (Tính hợp lý nếu có thể): (a) ( − 318 ) + 217 + 18 − 517 . (b) 27. ( 21 − 15 ) − 21. ( 27 − 15 ) .

a) \(\left( { - 318} \right) + 217 + 18 - 517\)

\( = \left( { - 318 + 18} \right) + \left( {217 - 517} \right)\)

\( = - 600\).

b) \(27.\left( {21 - 15} \right) - 21.\left( {27 - 15} \right)\).

\( = 27.21 - 27.15 - 21.27 - 21.\left( { - 15} \right)\)

\( = \left( {27.21 - 27.21} \right) - 27.15 + 21.15\)

\( = 0 + 15.\left( { - 27 + 21} \right)\)

\( = 15.\left( { - 6} \right) = - 90\).

c) \(\frac{7}{{12}} + \frac{{ - 14}}{{25}} + \frac{5}{{12}} + \frac{{ - 11}}{{25}}\)

\( = \left( {\frac{7}{{12}} + \frac{5}{{12}}} \right) + \left( {\frac{{ - 14}}{{25}} + \frac{{ - 11}}{{25}}} \right)\)

\( = 1 + \left( { - 1} \right) = 0\).