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# C1 Quadratic question Watch

1. This is the one thing I now don't understand and need some help with.

The question is :
Find the real roots of the equation x4 -5x2 -36 = 0 by considering it as a quadratic in x2

I square rooted the whole thing and ended up with the wrong answer using the quadratic formula on that rooted.

Help ):
2. (Original post by Dowel)
This is the one thing I now don't understand and need some help with.

The question is :
Find the real roots of the equation x4 -5x2 -36 = 0 by considering it as a quadratic in x2

I square rooted the whole thing and ended up with the wrong answer using the quadratic formula on that rooted.

Help ):
Let u = x^2. Then ....
3. Use substitution... let k=x^2
then solve as a normal quadratic then once you've found the values for k sub them into your substitution to gain the x values
4. (Original post by Dowel)
This is the one thing I now don't understand and need some help with.

The question is :
Find the real roots of the equation x4 -5x2 -36 = 0 by considering it as a quadratic in x2

I square rooted the whole thing and ended up with the wrong answer using the quadratic formula on that rooted.

Help ):
factoise like a quadratic

(x^2+4)(x^2-9)=0
X^2=0
x=3 , x= -3

to make easier, let y=x^2

(x^2)^2 - 5(x^2)-36
y^2 - 5y -36
(y+4)(y-9)
y=9
x^2 = 9
x=3 , x=-3
5. (Original post by greg2321)
factoise like a quadratic

(x^2+4)(x^2-9)=0
X^2=0
x=3 , x= -3

to make easier, let y=x^2

(x^2)^2 - 5(x^2)-36
y^2 - 5y -36
(y+4)(y-9)
y=9
x^2 = 9
x=3 , x=-3
This was really helpful, thanks

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