bonsoir
a = 123456789515
123456789514 = a-1
123456789513 = a-2
123456789512 =a-3
donc
(123456789515)²-(123456789514)²-(123456789513)²+(123456789512)²
a² -(a-1)²-(a-2)²+(a-3)²
a²-(a²-2a+1)-(a²-4a+4) +(a²-6a+9)
a² -a²+2a-1-a²+4a-4+a²-6a+9
a²-a²-a²+a²+2a+4a-6a-1-4+9
-1-4+9
-5+9
+4
voila bonne soiree