Factories the following expression-

81x²– 198 xy + 121y²

In the given expression

1^st and last terms are perfect square

⇒ 81x² = 9x × 9x

⇒ 121y² = 11y × 11y

And the middle expression is in form of 2ab

198xy = 2 × 9x × 11y

∴ 9x × 9x - 2 × 11y × 9x + 11y × 11y

Gives (a-b)² = a²-2ab + b²

⇒ In 81x² – 198xy + 121y²

a = 9x and b = 11y;

∴ 81x² – 198xy + 121y² = (9x-11y)²

Hence the factors of 81x² – 198xy + 121y² are (9x-11y)and (9x-11y)