Tính nhanh: a.   399 x 45 + 45 x 399 / 1995 x 1996 - 1991 x 1995

a. \(\frac{{399 \times 45 + 45 \times 399}}{{1995 \times 1996 - 1991 \times 1995}}\)     

\( = \frac{{399 \times \left( {45 + 55} \right)}}{{1995 \times \left( {1996 - 1991} \right)}} = \frac{{399 \times 100}}{{1995 \times 5}} = \frac{{399 \times 100}}{{399 \times 5 \times 5}} = \frac{{100}}{{25}} = 4\)

b. \(\frac{{1996 \times 1995 - 996}}{{1000 + 1996 \times 1994}}\)

\( = \frac{{1996 \times 1995 - 1996 + 1000}}{{1000 + 1996 \times 1994}} = \frac{{1996\left( {1995 - 1} \right) + 1000}}{{1000 + 1996 \times 1994}} = \frac{{1996 \times 1994 + 1000}}{{1000 + 1996 \times 1994}} = 1\)