Bonjour,
a) plutôt que (x+1)(x-1)-(x-2)(x-2), ne serait-ce pas (x+1)(x-1)-(x+2)(x-2) ?
(x+1)(x-1)-(x+2)(x-2) = x²-1²-(x²-2²)
= x²-x²-1+4
= 3
b) posons x = 296
297 = x+1
295 = x-1
298 = x+2
294 = x-2
donc 297 × 295 - 298 × 294 = (x+1)(x-1)-(x+2)(x-2) = 3