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Solve this quadratic =(?

  • Thread starter DeanBH
  • Start date
  • #1
82
0
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
 

Answers and Replies

  • #2
cristo
Staff Emeritus
Science Advisor
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Hint: Can you express y in terms of x, such that your second equation becomes your first?
 
  • #3
82
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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
Borek
Mentor
28,327
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Compare the equations term by term.

11=11

next is...
 
  • #5
82
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oh yeah, i can do that because they're both 0, and are of the same form. lewl
 
  • #6
tiny-tim
Science Advisor
Homework Helper
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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
Hint: substitution. :smile:
 
  • #7
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Hint: substitution. :smile:
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
tiny-tim
Science Advisor
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Bigger hint: substitute y = x². :smile:
 
  • #9
82
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Bigger hint: substitute y = x². :smile:

why can i just substitute that.
 
  • #10
tiny-tim
Science Advisor
Homework Helper
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why can i just substitute that.
eh? :confused: You can substitute anything you like. :smile:

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

But all substitutions are valid.

Try it … put y = x² into y - 8√y + 11 = 0, and see what happens! :smile:
 
  • #11
why can i just substitute that.
think about it a second:
you've already found that [tex]x^{2} - 8x + 11[/tex]
Now you need to find [tex]y - 8y^{1/2} + 11[/tex]
If you substituted [tex]x^{2}[/tex] = y,
you would have [tex]x^{2} - 8x + 11[/tex]
which you already have the answer to. If x = 4+/-[tex]\sqrt{5}[/tex],
what is [tex]x^{2}[/tex] (i.e. y) going to equal?
 

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