Factorise as far as possible into Prime Factors:

Question

5x^8 - 80y^4

Answer 1

first factor out a 5

5(x^8 - 16y^4)

now you should see a difference of squares

5(x^4 - 4y^2)(x^4 + 4y^2)
and once more

5(x^2 - 2y)(x^2 + 2y)(x^4 + 4y^4)

Answer 2

I believe Reiny meant to write
5(x^2 - 2y)(x^2 + 2y)(x^4 + 4y^2)
for the last step. The last y exponent is 2.

Answer 3

thanks drwls for catching that.

Time to get new reading glasses at the "Dollar Store", lol