Tính bằng cách hợp lý: A = 100 + 98 + 96 + + 2 - 99 - 97 - - 1

= (2+... + 96 + 98 + 100) – (1 +… + 95 + 97)

= \(\left[ {\left( {\frac{{100 - 2}}{2} + 1} \right)\left( {100 + 2} \right):2} \right]\) – \(\left[ {\left( {\frac{{97 - 1}}{2} + 1} \right)\left( {97 + 1} \right):2} \right]\)

= \(\frac{{\left( {5100{\rm{ }} - {\rm{ }}4802} \right){\rm{ }}}}{2}\)

= 149.