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#1 2012-12-19 13:01:58

Frublox
Member
Registered: 2012-12-19
Posts: 1

Square root problem

Got this problem on a test:
          ____
12 + √2x-1 = 4

Okay, so then I subtract 12 from both sides:
  ____
√2x-1 = -8
2x - 1 = 64   I then square both sides...
2x = 65        Add 1 to each side
x = 65/2       Divide both sides by two.

However, that answer doesn't work. Are there simply no solutions, then?

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#2 2012-12-19 14:21:46

bobbym
Administrator
From: Bumpkinland
Registered: 2009-04-12
Posts: 87,258

Re: Square root problem

Hi Frublox;

That is correct, there are no solutions. Not every equation is a true statement. You always check the validity of your answer by plugging into the original equation.

Please go here:

http://www.mathsisfun.com/algebra/radic … lving.html


In mathematics, you don't understand things. You just get used to them.
Of course that result can be rigorously obtained, but who cares?
Combinatorics is Algebra and Algebra is Combinatorics.

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#3 2012-12-19 20:45:43

bob bundy
Moderator
Registered: 2010-06-20
Posts: 6,427

Re: Square root problem

hi Frublox

That answer is the solution to

-12 + √(2x-1) = -4

When you square you cannot help but introduce 'solutions' to this alternative equation as well so you were right to check the value you had.

Bob


You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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#4 2012-12-19 21:25:17

anonimnystefy
Real Member
From: The Foundation
Registered: 2011-05-23
Posts: 15,544

Re: Square root problem

Even when you got √2x-1 = -8 you could've said that there are no solutions, because a square root of a positive number is always positive...


“Here lies the reader who will never open this book. He is forever dead.

“Taking a new step, uttering a new word, is what people fear most.” ― Fyodor Dostoyevsky, Crime and Punishment

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#5 2012-12-19 21:31:14

bobbym
Administrator
From: Bumpkinland
Registered: 2009-04-12
Posts: 87,258

Re: Square root problem

Hi;

Yes, it always takes the principal value.


In mathematics, you don't understand things. You just get used to them.
Of course that result can be rigorously obtained, but who cares?
Combinatorics is Algebra and Algebra is Combinatorics.

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