Evaluate:

(ab + x²) (ab - x²)

(ab + x²) (ab - x²)

Using the formula

[∵ (x + y)(x - y) = x² - y²]

= (ab)² - (x²)²

= a²b² - x⁴

Hence, (ab + x²) (ab - x²) = a²b² - x⁴