Math Is Fun Forum

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Two guys attempt to solve an equation.

HI;

A gives B this equation to solve:

He proceeds like this:

He plugs his answer back into the original problem like a good boy should.

and says 1 + 1 ≠ 0 so there is no solution to this equation.

A says why not check like this:

square both sides:

Of course B points out that by A's method you could prove that

3 + 3 = 0

3 = -3

9 = 9

So 3 + 3 does not equal 6 but 0. Now B is convinced he is right that there are no solutions to

Is he right? This actually took place.

Last edited by bobbym (2009-09-13 22:34:06)

---

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

mathsyperson

Moderator

Registered: 2005-06-22

Posts: 4,900

Re: Two guys attempt to solve an equation.

B is correct, and has even shown why A's logic doesn't work.

The equation doesn't have any real solutions, because no real number has a negative square root.

---

Why did the vector cross the road?
It wanted to be normal.

JaneFairfax

Member

Registered: 2007-02-23

Posts: 6,868

Re: Two guys attempt to solve an equation.

A says why not check like this:

square both sides:

When you solve an equation by squaring, you introduce extra solutions to the problem. The purpose of checking by substituting back is to eliminate any extra solutions. Squaring during checking just puts the extra solutions back and serves no purpose whatsoever.

Whoever A is, HE IS AN IDIOT!!

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Two guys attempt to solve an equation.

Hi Jane and mathsyperson;

Thanks, I am glad to hear that B is right.

Last edited by bobbym (2009-09-14 18:30:55)

---

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

phrontister

Real Member

From: The Land of Tomorrow

Registered: 2009-07-12

Posts: 4,835

Re: Two guys attempt to solve an equation.

I like that! And I can think of some candidates to try it out on!

Here's a link to a nice selection of invalid proofs:

http://en.wikipedia.org/wiki/Invalid_proof

---

"The good news about computers is that they do what you tell them to do. The bad news is that they do what you tell them to do." - Ted Nelson

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Two guys attempt to solve an equation.

Hi phrontister;

Yea, don't try it on B he is wise to it now. But he was a little flustered at first.

---

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

phrontister

Real Member

From: The Land of Tomorrow

Registered: 2009-07-12

Posts: 4,835

Re: Two guys attempt to solve an equation.

don't try it on B he is wise to it now...

B=YYITITITIT (yyuryyubicuryymemememe)

Last edited by phrontister (2009-09-14 21:17:55)

---

"The good news about computers is that they do what you tell them to do. The bad news is that they do what you tell them to do." - Ted Nelson