bonjour

2 x 4 - 3 ²

=  8 - 9 = - 1

7 x 9 - 8 ² = 63 - 64 = - 1

15 x 17 - 16 ² = 255 - 256 = - 1

( x - 1 ) ( x + 1) - x²

= x² - 1 - x²

= - 1