We know that (a – b) (a + b) = a2 – b2
Put a = x and b = y in the identity (1) then
(x – y) (x + y) = x2 – y2
Now (x – y)(x + y)(x2 + y2) = (x2 – y2) (x2 + y2)
Again put a = x2 and b = y2 in (1)
We have (x2 – y2) (x2 + y2) = (x2)2 – (y2)2
= x4 – y4
So (x – y) (x + y) (x2 + y2) = x4 – y4