Bonjour,
On a :
1. (x-2)(2x-3) - (2x - 4)(7-x) = (x-2)(2x-3) - 2(x-2)(7-x)
= (x-2)[(2x-3)-2(7-x)]
=(x-2)[2x-3-14+2x]
=(x-2)(4x-17)
2. On a (n-3)(n+3) = n² - 9
Donc si n²=256
(n-3)(n+3) = 256 - 9 = 247