Jump to content
BrainDen.com - Brain Teasers
  • 0

Am I wrong…2


wolfgang
 Share

Question

9 answers to this question

Recommended Posts

  • 0

  Reveal hidden contents

What happened to the underlined parts in the next step?

Edited by Thalia
Link to comment
Share on other sites

  • 0
  On 1/9/2012 at 10:17 PM, Peekay said:

  Reveal hidden contents

You are wrong on the 5th step where you casually mention canceling the powers on both sides!

(-1/2)^2 = (+1/2)^2 does not yield -1/2 = +1/2

But, definitely a good one!

  Reveal hidden contents

Can't you do it by taking the square root of both sides, which is a "legal" mathematical operation?"

Link to comment
Share on other sites

  • 0
  On 1/9/2012 at 10:47 PM, Thalia said:

  Reveal hidden contents

What happened to the underlined parts in the next step?

This step is legitimate. Think about squaring the term (X-Y). This gives you X^2-2XY+Y^2. In this case X=4 and Y = 9/2 so the square of (4-9/2) = 4^2 -2x4x9/2+(81)/9=4^2-4x9+(81)/4

  On 1/10/2012 at 12:21 AM, Morningstar said:

  Reveal hidden contents

Can't you do it by taking the square root of both sides, which is a "legal" mathematical operation?"

  Reveal hidden contents
When youy take the sqareroot, there are 2 possible roots since (-2)^2 and 2^2 both equal 4, when takeing the squareroot, we must consider which root is the appropriate. In this case note that the fith step which currently reads:

4- 9/2 = 5- 9/2

but by choosing the opposite sign on one side, we get instead

9/2-4=5-9/2

which indeed is a true statement since 1/2=1/2

Link to comment
Share on other sites

  • 0
  On 1/11/2012 at 1:28 PM, thoughtfulfellow said:

This step is legitimate. Think about squaring the term (X-Y). This gives you X^2-2XY+Y^2. In this case X=4 and Y = 9/2 so the square of (4-9/2) = 4^2 -2x4x9/2+(81)/9=4^2-4x9+(81)/4

  Reveal hidden contents

When youy take the sqareroot, there are 2 possible roots since (-2)^2 and 2^2 both equal 4, when takeing the squareroot, we must consider which root is the appropriate. In this case note that the fith step which currently reads:

4- 9/2 = 5- 9/2

but by choosing the opposite sign on one side, we get instead

9/2-4=5-9/2

which indeed is a true statement since 1/2=1/2

  Reveal hidden contents

The next step would be to take the square root of both sides, but because of examples like the one Peekay gave, you must take the plus or minus square roots of both sides. As someone with some teaching experience in math, this is a common misake of only taking whats known as the&nbsp;&nbsp;&quot;principle&quot; square root (only the positive one) of both sides.<br />

The issue in this case is the the expression on the left (when you said cancel the powers on both sides) (4-9/2)^2 is negative (-0.5 in this case) where as the right (5-9/2)^2 is positive (0.5 in this case).

EDIT: I didnt see the post that explained this already. Sorry for the second posting of the answer... :(

Edited by jaustinsmith_2005
Link to comment
Share on other sites

  • 0

This same question was found in Prof. Ramanujam's book and here goes the solution

(4-9/2)^2 = (5-9/2)^2

=> 4-9/2 = +/- (5-9/2)

Since 4 is not equal to 5, hence,

4-9/2 = -(5-9/2)

=> 4-9/2 = -5 + 9/2

=> 4+5 = 9/2 + 9/2

=> 9 = 9

Left Hand Side = Right Hand Side

Good luck with your maths exam

I tried to use Spoiler, but it is not working in my machine for reasons i dont know. Hence posted the solution publicly.

Edited by Satish.S
Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Answer this question...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...