Question 1096673
x = a-b and y = b-c, then
:
a = x+b, b = y+c and c = b-y
:
(a-c)(a+c-2b) = (x+b-(b-y))(x+b-b-y-2b) =
:
(x+y)(x-y) = x^2-y^2
: