Callisto
  • Callisto
Tutor question #3 Simplify: \[\sqrt{5-\sqrt{21}}\] (conditions: pen, paper, no calculator, done in 2 minutes) *
Mathematics
  • Stacey Warren - Expert brainly.com
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SOLVED
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katieb
  • katieb
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lgbasallote
  • lgbasallote
what if x = \(\sqrt{5-\sqrt{21}}\) then \(x^2 = 5 - \sqrt{21}\) \[x^2 - 5 = -\sqrt{21}\] \[(x^2 - 5)^2 = 21\] oh goodness quadratic o.O
anonymous
  • anonymous
actually it is a 4th degree equation
Callisto
  • Callisto
\[(x^2-5)^2=21\]\[x^4-10x^2+25=21\]\[x^4-10x^2+4=0\]\[x^4-10x^2+4=0\]\[x^2=\frac{10\pm \sqrt{(-10)^6-4(4)}}{2}\]\[x^2=5\pm \sqrt{21}\]Sorry... are you sure that it is 'simplified'?

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More answers

Callisto
  • Callisto
Oh.that's power 2, typo
anonymous
  • anonymous
apparently it is also \[\sqrt{\frac{7}{2}}-\sqrt{\frac{3}{2}}\] i remember seeing something like this before but i am not sure i remember how to go from one to the other
Callisto
  • Callisto
I need to know how to work that out.....
anonymous
  • anonymous
cant seem to get it, i think it was a trick
Callisto
  • Callisto
............................ It... was ... a ... question ... asked ... when ... my friend applied for a summer job ...
anonymous
  • anonymous
really? doing what??
Callisto
  • Callisto
tutor - teaching high school students
anonymous
  • anonymous
hmm i wonder what answer they wanted
Callisto
  • Callisto
Same here...
Callisto
  • Callisto
When I was doing some exercises few days ago, I saw similar questions, but clearer, like this: express \(\sqrt{28-2\sqrt{147}}\) in the form of \(\sqrt{x}-\sqrt{y}\). I can still handle this. But that one, I failed :(
anonymous
  • anonymous
multiplying by the conjugate give \(\frac{2}{\sqrt{5+\sqrt{21}}}\) think
myininaya
  • myininaya
any examples anywhere?
Callisto
  • Callisto
example?!
myininaya
  • myininaya
You want to show \[\sqrt{5-\sqrt{21}} \text{ equals } \frac{\sqrt{7}-\sqrt{3}}{2} ?\] was just wondering if you have an example for writing that one thing in that other form
myininaya
  • myininaya
oops sqrt(2) on bottom
myininaya
  • myininaya
how did you get that identity?
Callisto
  • Callisto
From experience...
myininaya
  • myininaya
lol great experience I want to prove that identity lol
Callisto
  • Callisto
First, \((\sqrt{a} - \sqrt{b})^2\) = a + b -2\sqrt{ab} For a>b \[\sqrt{5-\sqrt{21}} = \sqrt{5-2\sqrt{\frac{21}{4}}}\] Now, a+b = 5 => a=5-b ab = 21/4 (5-b)b = 21/4 -4b^2 + 20b - 21 =0 b=1.5 or b =3.5 (rejected) a = 5 - 1.5 = 3.5 So, it is \(\sqrt{\frac{7}{2}}-\sqrt{\frac{3}{2}}\) Does that make sense?
myininaya
  • myininaya
ok i see that identity :)
myininaya
  • myininaya
that one is easy to prove
myininaya
  • myininaya
maybe because it is an actual identity, right? :p
Callisto
  • Callisto
Perfect square is perfect :)
Callisto
  • Callisto
Does that make sense? Apart from the latex fail...
myininaya
  • myininaya
very interesting i wouldn't have thought of that
Callisto
  • Callisto
I'm going to post the link to satellite73's post then. He doesn't even come to check..
myininaya
  • myininaya
Great work @Callisto :)
Callisto
  • Callisto
And thank you for all your time!!!!

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