I tried some cases, like with n= 4, k = 3, and i = 1, and i got like -4 + 12 -12 +4 = 0 (seems symeetrical), but I am not sure where to go to actually prove this identity.
I tried some cases, like with n= 4, k = 3, and i = 1, and i got like -4 + 12 -12 +4 = 0 (seems symeetrical), but I am not sure where to go to actually prove this identity.