a² - (x-4)² = a² - (x² - 8x + 16)

= a² - x² + 8x - 16

= a² - x² + (8x - 16)

Decreasing x by 4 causes (a² - x²) to be increased by (8x-16) ............ANS

a² - x² vs a² - (x-4)². What is the difference of these two expressions?