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catgmat
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PowerPrep: Radicals Updated on: 24 Jun 2007, 12:04
Originally posted by catgmat on 23 Jun 2007, 00:27.
Last edited by catgmat on 24 Jun 2007, 12:04, edited 2 times in total.
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Please explain, OA is E! Cheers



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catgmat
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PowerPrep: Radicals 24 Jun 2007, 02:45
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Could someone try to explain, please? cheers
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AugiTh
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PowerPrep: Radicals 24 Jun 2007, 11:16
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put 9 as sqrt(81) and then use (a+b)^2 = a^2 + b^2 + 2ab formula

you get
sqrt(81) + sqrt(80) +sqrt(81) -sqrt(80) + 2*sqrt(sqrt(81)+sqrt(80))*(sqrt(sqrt(81)-sqrt(80)) -> eq1

sqrt(sqrt(81)+sqrt(80))*(sqrt(sqrt(81)-sqrt(80)) is in
(a+b)*(a-b)=a^2-^b^2
Therefore sqrt(sqrt(81)+sqrt(80))*(sqrt(sqrt(81)-sqrt(80))
= sqrt(81-80) =1

thus eq1 becomes

sqrt(81) +sqrt(81) + 2

which gives 9+9+2 =20

hope this helps
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catgmat
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PowerPrep: Radicals 24 Jun 2007, 13:18
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AugiTh
put 9 as sqrt(81) and then use (a+b)^2 = a^2 + b^2 + 2ab formula

you get
sqrt(81) + sqrt(80) +sqrt(81) -sqrt(80) + 2*sqrt(sqrt(81)+sqrt(80))*(sqrt(sqrt(81)-sqrt(80)) -> eq1

sqrt(sqrt(81)+sqrt(80))*(sqrt(sqrt(81)-sqrt(80)) is in
(a+b)*(a-b)=a^2-^b^2
Therefore sqrt(sqrt(81)+sqrt(80))*(sqrt(sqrt(81)-sqrt(80))
= sqrt(81-80) =1

thus eq1 becomes

sqrt(81) +sqrt(81) + 2

which gives 9+9+2 =20

hope this helps
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thanks a lot mate! now I got it. cheers
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ian7777
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PowerPrep: Radicals 24 Jun 2007, 14:37
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this is kind of crazy, but i did it differently.

WIthout making 9 root 81, i squared the same way, and we end up with

18 + 2*Root(9+root80)*root(9-root80)

Wasn't sure what to do with the second part there, so I just squared the whole thing, that way I'd get rid of the outter radicals, figuring I could square root it again when I was done.

Turns out, I was right, because when you do that, you just get 4. Then I square rooted it again back to 2, and then you've just got

18+2

20



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