bonsoir
K = ( x + 15 )² - ( x - 15 )²
x² + 30 x + 225 - ( x² - 30 x + 225 )
x² + 30 x + 225 - x² + 30 x - 225
= 60 x
1 215 ² - 1 185² = ( 1 200 + 15 )² - ( 1 200 - 15 ) ²
= 60 * 1200 = 72 000
