(1,0 điểm) Tính hợp lí biểu thức sau: a) − − ( − 271 ) − − ( 531 + 371 − − 731 ) . b) 87 ⋅ ( 13 − 45 ) − 13 ⋅ ( 87 + 45 ) .

a) \[--\left( { - 271} \right)--\left( {531 + 371--731} \right)\]

\( = 271--531 - 371 + 731\)

\( = \left( {271 - 371} \right) + \left( {--531 + 731} \right)\)

\( = \left( { - 100} \right) + 200\)

\( = 100.\)

b) \[87 \cdot \left( {13 - 45} \right) - 13 \cdot \left( {87 + 45} \right)\]

\[ = 87 \cdot 13 - 87 \cdot 45 - 13 \cdot 87 - 13 \cdot 45\]

\[ = \left( {87 \cdot 13 - 13 \cdot 87} \right) - \left( {87 \cdot 45 + 13 \cdot 45} \right)\]

\[ = - 45 \cdot \left( {87 + 13} \right)\]

\[ = - 45 \cdot 100\]

\( = - 4\,\,500.\)