# Math Help - Factorise

1. ## Factorise

Factorise y4 - 3y^2 - 4. Hint. Let z = y^2

2. Originally Posted by AJShaw
Factorise y4 - 3y^2 - 4. Hint. Let z = y^2
$y^4 - 3y^2 -4 = (y^2)^{2} - 3y^2 -4$

if $z=y^2$, your problem becomes:

$z^2 - 3z -4$

Try factorizing now

3. I am just learning factorising. Is it:

z^2-3z-4

z^2-3z-4

z-3-4

z-7

If not right, can you show me how to do it please?

4. Originally Posted by AJShaw
I am just learning factorising. Is it:

z^2-3z-4

z^2-3z-4

z-3-4 : How did you get this?

z-7

If not right, can you show me how to do it please?

$z^2-3z-4$

$= z^2-(4-1)z - 4$

$= z^2 - 4z + z - 4$

$= z(z-4) + 1(z-4)$

$=(z+1)(z-4)$

Now substitute the value of z that you had assumed at the beginning of the quesiton.