So sánh: A = (-9).(-3) + 21.(-2) + 25 và N = (-5).(-13) + (-3).(-7) - 80

\(A = \left( { - 9} \right).\left( { - 3} \right) + 21.\left( { - 2} \right) + 25\)và \(B = \left( { - 5} \right).\left( { - 13} \right) + \left( { - 3} \right).\left( { - 7} \right) - 80.\)

Ta có:   \(A = \left( { - 9} \right).\left( { - 3} \right) + 21.\left( { - 2} \right) + 25\)\( = 27 - 42 + 25\,\, = \,\,10\)

              \(B = \left( { - 5} \right).\left( { - 13} \right) + \left( { - 3} \right).\left( { - 7} \right) - 80.\)\( = 65 + 21 - 80\,\, = \,\,6\)

Vì \(10\,\, > \,\,6\) , suy ra \(A\,\, > \,\,B\)