Ta có:
` 1995 × 1995 = 1995 × (1991 + 4) = 1995 × 1991 + 1995 × 4 `
` 1991 × 1999 = 1991 × (1995 + 4) = 1995 × 1991 + 1991 × 4 `
Vì ` 1995 × 1991 + 1995 × 4 > 1995 × 1991 + 1991 × 4 `
` -> 1995 × 1995 > 1991 × 1999 `
Lại có:
` 1 = \frac{1991×1999}{1991×1999} `
Mà ` 1995 × 1995 > 1991 × 1999 `
` => \frac{1995×1995}{1991×1999} > \frac{1991×1999}{1991×1999} `
` => \frac{1995×1995}{1991×1999} > 1 `
Vậy ` \frac{1995×1995}{1991×1999} > 1 `
