# Homework Help: Solve this quadratic =(?

1. May 3, 2008

### DeanBH

first part of the question is simple :
x^2 - 8x + 11 = 0

solve. using quadratic formula it is 4 +/- root5

second part confuses me, you are given to equation:

y - 8y^(1/2) + 11 = 0

and are told to :

solve this giving answer in form p +/- Q * root5

i have no idea how to do this. the quadratic formula doesnt work on this one and i dont understand how part 1 of this question helps me with this part, can someone explain it to me please.!? thanks

2. May 3, 2008

### cristo

Staff Emeritus
Hint: Can you express y in terms of x, such that your second equation becomes your first?

3. May 3, 2008

### DeanBH

i don't know how to do that, i've tryed making the equations equal to each other but it doesn't work out right

4. May 3, 2008

### Staff: Mentor

Compare the equations term by term.

11=11

next is...

5. May 3, 2008

### DeanBH

oh yeah, i can do that because they're both 0, and are of the same form. lewl

6. May 3, 2008

### tiny-tim

Hint: substitution.

7. May 3, 2008

### DeanBH

i've honastly tried for ages, i still dont know how to do it.

can someone just run me through it. it's not even a question for homework or anything im just revising and i dont understand this.

8. May 3, 2008

### tiny-tim

Bigger hint: substitute y = x².

9. May 3, 2008

### DeanBH

why can i just substitute that.

10. May 4, 2008

### tiny-tim

eh? You can substitute anything you like.

Some substitutions make the problem easier :!!) , some substitutions make it harder.

But all substitutions are valid.

Try it … put y = x² into y - 8√y + 11 = 0, and see what happens!

11. May 4, 2008

### Dr Zoidburg

you've already found that $$x^{2} - 8x + 11$$
Now you need to find $$y - 8y^{1/2} + 11$$
If you substituted $$x^{2}$$ = y,
you would have $$x^{2} - 8x + 11$$
which you already have the answer to. If x = 4+/-$$\sqrt{5}$$,
what is $$x^{2}$$ (i.e. y) going to equal?