Ta có a^2 - b^2 = (a + b),(a - b) (theo kết quả bài 9 - Dạng 3)

Ta có : \[9876543{\rm{ }}.{\rm{ }}9876545\; = {\rm{ }}\left( {9876544{\rm{ }}--{\rm{ }}1} \right)\left( {9876544{\rm{ }} + 1} \right)\;\;\]

                                             = \({9876544^2} - {1^2}\)

 Vì \({9876544^2} - {1^2}\)< \({9876544^2}\) nên \(9876543{\rm{ }}.{\rm{ }}9876545{\rm{ }} < {9876544^2}\)