Square root!

Algebra Level 1

True or False?

If ( a b ) × ( a b ) = ( c b ) × ( c b ) , (a-b) \times (a-b) = (c-b) \times (c-b), then it must be true that a = c . a=c.

Bonus: If you think it's true, prove it; if you think it's false, find a counterexample.

True False

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59 solutions

Zico Quintina
May 27, 2018

Relevant wiki: Quadratic Equation

( a b ) 2 = ( c b ) 2 ( a b ) = ± ( c b ) a = c OR a = 2 b c . (a - b)^2 = (c - b)^2 \implies (a - b) = \pm (c - b) \implies a = c \ \text{ OR } \ a = 2b - c.

I don't quite follow the different equivalents.

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Two possibilities:

a b = c b a = c a-b = c-b \implies a=c

a b = ( c b ) a b = c + b a = 2 b c a-b = -(c-b) \implies a-b = -c+b \implies a = 2b-c

Zain Majumder - 3 years ago

How can I add counterexample??

Dzhoel Stark - 3 years ago

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take the formular from above and fill in any 2 of the three numbers. Lets take b=2 and c=3 this will give us: a = 2*2 - 3 = 1

Minkehr . - 3 years ago

So basically it means either a=c or a,b,c are in arithmetic progression.

Anurag Pandey - 3 years ago

(a-b)^2 ) = (c-b)^2 a^2 -2ab + b^2 = c^2 -2cb + b^2 a^2 -2ab = c^2 - 2cb -b^2+ b^2 a^2 -2ab = c^2 - 2cb a^2 = c^2 -2cb + 2ab c^2 = a^2 +2cb -2ab a^2 + c^2 = c^2 + a^2 0 = 0 so a = c

Nuno Andrade - 3 years ago

Take ( 2 3 ) 2 (2-3)^{2} and ( 4 3 ) 2 (4-3)^{2}

( 2 3 ) 2 = ( 1 ) 2 = 1 (2-3)^{2} = (-1)^{2} = 1 and ( 4 3 ) 2 = 1 2 = 1 (4-3)^{2} = 1^2=1

Here 2 is not equal to 4, so this is an example for the above statement to be wrong.

If we take ( 2 3 ) 2 (2-3)^2 and ( 2 3 ) 2 (2-3)^2 ,

Now this an example for the above equation to be true.

So, the correct answer would be True, sometimes(or False).

Moderator note:

Note when featuring the problem we decided to simplify it into a True/False, so it isn't necessary to decide cases where the statement works and cases where it doesn't. It's now, simply using the logic of an if-then statement, the conclusion always follows when the conclusion is true (making the if-then statement true) or the conclusion doesn't always follow (making the if-then statement false).

So, can we add an extra option ""sometimes true". Or can we add a condition that a , b , c > 2 a, b, c > 2

Ram Mohith - 3 years ago

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If a,b,c > 2, we can still find a counter-example.

a = -10 b = 10 c = 30 this also works and here a,b,c > 2

Raphael d'Orsay - 3 years ago

In mathematics, "sometimes true" means "not true". For example, the statement "the product of any two natural numbers is even" is false, even though sometimes such a product is even.

Michael Mendrin - 3 years ago

The question asks for, " must be true." So if the answer is sometimes true, then you should still answer the question as "false" because it's not always true.

Laura Gao - 3 years ago

As Laura said, the logics of the question make the only correct answer „FALSE”.

Vlad Padina - 3 years ago

one single counter example is enough to prove that the statement is false, in other words, to say that it is "sometimes true and sometimes false" is to say that it is false.

Raphael d'Orsay - 3 years ago

@Vaibhav Priyadarshi , Please don't ask a confusing question the answer of which is ambiguous.

Winod DHAMNEKAR - 1 year, 7 months ago
Inksa Inkeroinen
May 27, 2018

Counterexample: b = 0 , a = c b=0, a=-c ( a 0 ) (a\neq0)

Then, ( a b ) × ( a b ) = a 2 = ( c ) 2 = c 2 (a-b)\times(a-b)=a^2=(-c)^2=c^2 and obviously ( c b ) × ( c b ) = c 2 (c-b)\times(c-b)=c^2

a = 2 b = 4 c = 6

(2 - 4) x (2 - 4) = (6 - 4) x (6 - 4)

-2 x -2 = 4

2 x 2 = 4

Gavin Downes - 3 years ago

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Nice and clear explanation

Jasper Butcher - 2 years, 10 months ago

Could this be put into more abstract terms?

In other words, can we derive a full equivalence class for these values in the equation?

Joshua Nesseth - 2 years, 8 months ago

Is this the only class of counter example possible?

Aisthu Lucky - 2 years, 11 months ago
Piero Sarti
May 28, 2018

If we expand the parentheses:

( a b ) 2 = ( c b ) 2 a 2 2 a b + b 2 = c 2 2 b c + b 2 (a-b)^2=(c-b)^2 \implies a^2 -2ab + b^2 = c^2 - 2bc + b^2

a 2 c 2 = 2 a b 2 b c a^2 -c^2 = 2ab -2bc

( a + c ) ( a c ) = 2 b ( a c ) a + c = 2 b (a+c)(a-c)=2b(a-c) \implies a+c = 2b

Although the statement holds for a = c a=c it also holds for a + c = 2 b a+c=2b .

Piero, in order to get a+c=2b, you need to simplify both sides dividing by (a-c). You can only divide it as long as a is different than c.

Rogerio De Souza - 3 years ago
Mitchel Paulin
May 27, 2018

The easiest solution is to take a = 1, c = -1 and b = 0. Then 1 x 1 = -1 x -1 = 1

Naren Bhandari
May 27, 2018

( a b ) 2 = ( c b ) 2 ( b a ) 2 = ( c b ) 2 2 b = a + c (A.M) (a-b)^2 = (c-b)^2 \\ (b-a)^2 = (c-b)^2 \implies 2b = a+c \ \text{(A.M)} shows that a c a \neq c . Hence the statement we have is false . The statement will be true either a = b a = b or b = c b=c .


( a b ) 2 = ( c b ) 2 (a-b)^2= (c-b)^2 doesn't implies a b = c b a-b = c-b however, ( a b ) = c b |(a-b)| = |c-b| .

Put a=2 and b=1 and c=2 so according to equation 1=1

Harpreet Singh - 2 years, 11 months ago
Oon Han
May 28, 2018

If we take b = 0, a and c may not necessarily be equal, as if a^2 = c^2, then a = c OR a = -c.

Uros Stojkovic
May 28, 2018

We have:

( a b ) 2 = ( c b ) 2 a 2 2 a b + b 2 = c 2 2 c b + b 2 a 2 c 2 2 b ( a c ) = 0 ( a c ) ( a + c 2 b ) = 0 \begin{aligned} \left ( a-b \right )^{2} &= \left ( c-b \right )^{2} \\ a^{2} - 2ab + b^{2} &= c^{2} - 2cb + b^{2} \\ a^{2} - c^{2} - 2b\left ( a-c \right ) & = 0 \\ \left ( a-c \right )\left ( a+c-2b \right ) &= 0\end{aligned}

So, it's either a = c a = c or a + c = 2 b a + c = 2b and thus the answer is False. \boxed{\text{False.}}

It's (a-b)^2 and you wrote the formula of (a+b)^2

Aaqil Shaik - 3 years ago

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Thanks, I changed it.

Uros Stojkovic - 3 years ago
Utkarsh Jha
May 27, 2018

We have (a-b) (a-b) = (c-b) (c-b)--------eq1, This simplifies to a^2-2ab=c^2-2bc. Solving further we get the relation a+c=2b. This clearly indicates that a,b,c are in arithmetic progression. Therefore eq1 is true for any three consecutive terms in an arithmetic progression . Thus, this implies that eq1 is not only true for a=c but also for other values of a and c. For example, let the A.P. be 2,5,8 where a=2, b=5 and c=8, Now, (2-5) (2-5)=(8-5) (8-5) 9=9 Since LHS=RHS, the above equality holds true.

Ram Mohith
May 27, 2018

Let us take a = 5 , b = 3 , c = 1 a = 5 ,b = 3 ,c = 1

( a b ) 2 = ( 5 3 ) 2 = 2 2 = 4 (a - b)^2 = (5 - 3)^2 = 2^2 = 4

( c b ) 2 = ( 1 3 ) 2 = ( 2 ) 2 = 4 (c - b)^2 = (1 - 3)^2 = (-2)^2 = 4

Here, a c a \neq c but ( a b ) × ( a b ) = ( c b ) × ( c b ) (a - b) \times (a - b) = (c - b) \times (c - b)


If we observe carefully here the condition a c a \neq c is satisfied only when a,b,c are in A.P .

Now put a = a, b = a + d, c = a + 2d

When we substitue the above in ( a b ) × ( a b ) = ( c b ) × ( c b ) (a - b) \times (a - b) = (c - b) \times (c - b) we get d 2 = d 2 d^2 = d^2 .

This tells that our assumption that a,b,c are in A.P is true. So, it is obvious that a c a \neq c .

Frank Caron
Jun 3, 2018

Let A = 1, B = 0, C = -1. The result will be 1 = 1 through the formula, yet a != c.

Mat Menzies
Jun 3, 2018

counter example, a=1, b=2, c=3.

Jon Syla
Jun 3, 2018

Not necessarily a=c because: (a-b)^2=(c-b)^2 ; (a-b)^2-(c-b)^2=0 ; [(a-b)-(c-b)][(a-b)+(c-b)]=0 ; (a-c)(a+c-2b)=0 therefore either a-c=0 or a+c-2b=0 where we get: a=c or a=2b-c.
So the answer is false.

Sean Demers
Jun 2, 2018

Here is a proof by counterexample

Let b = 0 b = 0

Our equation then simplifies to
a 2 = c 2 a^2 = c^2

Which has a = c a = -c as a solution.

Therefore, a is not necessarily equal to c in all cases. Thus, the answer is False .

Joseph Quarcoo
Jun 2, 2018

(7-3)(7-3)=(-1-3)(-1-3) and 7 \neq -1

Joshua Healey
Jun 2, 2018

( a b ) 2 = ( c b ) 2 = > ( 2 3 ) 2 = ( 4 3 ) 2 = > ( 1 ) 2 = ( 1 ) 2 = > a = 2 b c (a-b)^2 = (c-b)^2 => (2-3)^2 = (4-3)^2 => (-1)^2 = (1)^2 => a = 2b - c or ( a b ) 2 = ( c b ) 2 = > m 2 = m 2 (a-b)^2 = (c-b)^2 => m^2 = -m^2 therefore, a = c a = c is not required since we can achieve m and -m with different values for a and c.

For a counterexample, simply let b = 0 and we have the classic assumption where two squares equal means their square roots are also equal. Clearly FALSE.

Adam Weishaupt
Jun 2, 2018

This is the same as the top solution I just wanted to point out that the simplified equation is ±(a - b) = ±(c - b) NOT (a - b) = ±(c - b), either side of the equation may be positive or negative since the square root must be taken of both sides.

Achyut Dwivedi
Jun 2, 2018

Basically the question asks us if the f ( x ) = ( x b ) 2 f(x) = (x-b)^2 is an injective function or not. It can simply be checked graphically by drawing y = ( x b ) 2 y = (x-b)^2 for an arbitrary value of b b and if any line parallel to x-axis, y = k y = k cuts the graph at two or more points it's not an injective function.

This clearly shows that f ( x ) = ( x b ) 2 f(x) = (x-b)^2 is not an injective function hence for the original question, if ( a b ) × ( a b ) = ( c b ) × ( c b ) (a-b) × (a-b) = (c-b) × (c-b) the statement a = c a = c may not be true.

The only "exception" is the case ( a b ) × ( a b ) = ( c b ) × ( c b ) = 0 (a-b) × (a-b) = (c-b) × (c-b) = 0 , where the statement a = c = b a = c = b must be true

Jonathan Fudge
Jun 1, 2018

When it comes down to it, I was able to simplify to: a^2 - a = c^2 - c

I used a=3, then solved for what c could be. It could be 3 OR -2. Therefore, since one solution was found, then statement is false.

Rogerio De Souza
Jun 1, 2018

if you divide left and right parts by (a+b) would you get that a is not -b?

Max Vetrov - 3 years ago

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Max, you cannot simplify the right side with a-c, because this can ONLY be done as long as a is not equal to c.

Rogerio De Souza - 3 years ago

If a=c than you get that c may be any number you want.

Max Vetrov - 3 years ago

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Max, if a=c then you will get 0 at denominator, which makes it impossible.

Rogerio De Souza - 3 years ago
Richard Desper
Jun 1, 2018

Counter example: b = 0 , a = c 0 b = 0, a = -c \neq 0 .

Karen Utsey
Jun 1, 2018

( a b ) ( a b ) = ( c b ) ( c b ) (a-b)(a-b)=(c-b)(c-b) , let a = 2 and b = 3 results in an imaginary solution

Ashley Mitchell
May 31, 2018

(2-1)x(2-1) = (0-1)x(0-1)

Vipul Shukla
May 31, 2018

If a = c or a + c = 2b then the above equation is true. Therefore it is not necessary that a must be equal to c

Leon Wheeler
May 31, 2018
( a b ) 2 = ( c b ) 2 (a-b)^2 = (c-b)^2 N o w . . . L e t : a = c . . . Now...Let: a=c...
a 2 2 a b + b 2 = c 2 2 b c + b 2 a^2 - 2ab + b^2 = c^2 - 2bc + b^2 ( a b ) 2 > ( c b ) 2 (a-b)^2 -> (c-b)^2
a 2 2 a b = c 2 2 b c a^2 - 2ab = c^2 - 2bc ( c b ) 2 = ( c b ) 2 (c-b)^2 = (c-b)^2
a 2 c 2 = 2 a b 2 b c a^2 - c^2 = 2ab - 2bc ( c b ) 2 ( c b ) 2 = 0 (c-b)^2 - (c-b)^2 = 0
a 2 c 2 = 2 b ( a c ) a^2 - c^2 = 2b(a - c) . . . I t i s f a l s e . . . ...It is false...
( a + c ) ( a c ) a c = 2 b \frac{(a+c)(a - c)}{a-c} = 2b
a = 2 b c a = 2b - c
Cosmin Mazilu
May 31, 2018

If a=b+x and c=b-x the statement is always true, no matter the value of x.

Izzy Reuber
May 31, 2018

Quadratic equations tend to have to different roots/solutions, due to their u-shaped graph

Vitor Cesario
May 30, 2018

(a - b)² = (c - b)²

  • a² - 2ab + b² = c² - 2cb + b²

  • a² - c² = 2ab - 2cb

  • a² - c² = 2b(a - c)

  • a ² c ² a c \frac{a² - c² }{a - c} = 2b

  • ( a + c ) ( a c ) ( a c ) \frac{(a + c) (a - c)}{(a - c)} = 2b

  • a = 2b - c

Ribbitt Iii
May 30, 2018

(-1x2)^2=(1x2)^2

Tony Pelletier
May 30, 2018

since the equation could be seen as x 2 = y 2 x^2=y^2 any values that satisfy a b = ( c b ) a-b=-(c-b) or a = 2 b c a=2b-c will work.

Brody Burkett
May 30, 2018

The answer must be false on technicality, as you reduce the statement to a^2 - 2ab = c^2 - 2cb. You can use the quadratic formula to show that c will have 2 values, therefore not always equal to a.

Jonathan Burros
May 30, 2018

a=1 b=2 c=3 a not equal to c

Azzam Alsharafi
May 30, 2018

( a b ) × ( a b ) (a-b) \times (a-b) is the same thing as ( a b ) 2 (a-b)^{2} , and as we know the square of a positive number is the same as the square of it's negative value. So ( a b ) 2 = ( c b ) 2 (a-b)^{2} = (c-b)^{2} is possible if ( a b ) = ( c b ) (a-b) = -(c-b) .

Example:

( 2 3 ) = ( 4 3 ) (2-3) = -(4-3)

1 = ( 1 ) -1 = -(1)

1 2 = 1 2 -1^{2} = 1^{2}

a^2 - b^2 = c^2 - b^2, a^2 = c^2, a = c or a = - c

Anubhav Pal
May 29, 2018

Simply take b=0 and a=1 so b may be equal to-1 hence statement is false

Anunay Chandra
May 29, 2018

Given that a minus b square is equal to C minus b square . Now here 2 cases arise : Case 1: A minus b = C minus b in this case the value of a is equal to that of c Case 2: A minus b = negative of C minus b that is a minus b = B minus C it implies that A plus c Is equal to twice of B so in this case a is not equal to c. Example : A is equal to 1 B = 2 and C = 3

Take b = 0 b=0 , since it doesn't matter for the purposes of finding a counterexample.

Now we just have

a 2 = c 2 a^2=c^2

But we know that square roots can have both positive and negative solutions, so taking the square root of both sides produces:

± a = ± c \pm a=\pm c

Because the signs can differ on both sides, a doesn't have to equal c -- it can also be the negative of c, and when they're squared, they become equal.

Example:

a = 3 , b = 0 , c = 3 a=3,b=0, c=-3

3 2 = ( 3 ) 2 3^2 = (-3)^2

Bruno Seemann
May 29, 2018

If a is equal to 2 and b is equal to 1 then the result of 1x1 (that is the end of the a-b count) will be 1. But if c is 0 then the solution will be (0 - 1) x (0 - 1) that is (-1) x (-1), this is equal to 1 so the afirmative inst always right

Supper King
May 29, 2018

if a = -2 and b = 2 then the answer to the first is 16. then C could be 6 and b is again 2 then the answer is 16

Jordan Campbell
May 29, 2018

Counter example: If you let a=0 then you can show that c=2b. so a=0 b=1 c=2

Heinz Backyard
May 29, 2018

If b=0 we end up with a² = c² which can be achieved by 1² = (-1)², here a =/= c therefore the above statement does not hold true.

Marcus Garrett
May 29, 2018

If a^2 - 2ab + b^2 = c^2 -2cb + b^2, Then a^2 - c^2 = 2ab – 2cb, and a^2 - c^2 = 2b (a – c). So, (a + c)(a – c) = 2b (a – c), Thus a + c = 2b, that is, a = 2b - c

Faris Firdaus
May 29, 2018

At this point, it is clear that the expression can't be simplified anymore. Hence, a ≠ c

This is not correct explanation.

Vaibhav Priyadarshi - 3 years ago
Rajen Ganeshrenu
May 28, 2018

Counter example is (3-4) (3-4)=(11-10) (11-10).... There are a lots of example such that a is not equal to c.

It is not a counterexample. Question is asking for (a-b)(a-b)=(c-b)(c-b). But you have taken two values of b which we can't take because a particular variable denotes Same value in same condition.

Vaibhav Priyadarshi - 3 years ago
  • a x - a its equal to a x a, then a is not equal to -a
Spaid Anonamoys
May 28, 2018

If a=c then that means there would be no reason to have a c unless we were comparing natural to ordinals which in a logical sense would not be logical you could be taking absolutes but no one like those

Joshua Clymer
May 28, 2018

(2-1)^2 = (0-1)^2

Rianne Niemeijer
May 28, 2018

( a - b )^2 = ( c - b )^2

So how about using the following:

a = -2

b = 4

c = 10

Would make: ( -2 -4 )^2 = ( 10 - 4 )^2

-2 -4 = -6

-6^2= 36

10 - 4 = 6

6^2 = 36

36 = 36

It's a simple matter of - x - = +

(a)(a)+(a)(−b)+(−b)(a)+(−b)(−b)+−b2+2bc+−c2

=a2+−ab+−ab+b2+−b2+2bc+−c2

Combine Like Terms:

=a2+−ab+−ab+b2+−b2+2bc+−c2

=(a2)+(−ab+−ab)+(b2+−b2)+(2bc)+(−c2)

=a2+−2ab+2bc+−c2

Matt Miguel
May 28, 2018

Conjugates are indistinguishable for polynomials of real coefficients. Simple example is b=0 a=i c=-i

Joe Thompson
May 28, 2018

Imagine a parabola ( x b ) 2 x-b)^{2} , it is symmetric around x = b x=b so for every d > 0 d>0 there will be two values of x x where ( x b ) 2 = d x-b)^{2}=d is satisfied

Rahul Singh
May 28, 2018

Take a = 1 ; b = 2 ; c = 3 a=1;b=2;c=3 (where a!=c) Put in above equation: ( 1 2 ) ( 1 2 ) = ( 3 2 ) ( 3 2 ) (1-2)*(1-2)=(3-2)(3-2) which implies that: ( 1 ) 2 = ( 1 ) 2 (-1)^2 = (1)^2 Therefore: 1 = 1 1=1

Edwin Gray
May 28, 2018

let a =1, c =0, b= .5. then a -b = .5 and c -b = -.5, and (a -b)x(a -b) = .25, and (c - b)x(c - b -.5x-.5 = .25Ed Gray

Jay Zirroh
May 28, 2018
  • a2 - 2ab + b2 = c2 - 2bc + b2
  • a2 - 2ab = c2 - 2bc
  • a(a - 2b) = c(c - 2b)
  • a = c(c - 2b)/(a - 2b)
  • a /= c
Uttam Manher
May 28, 2018

If ( a b ) ( a b ) = ( c b ) ( c b ) (a - b)(a - b) = (c - b)(c - b)

then ( a b ) 2 = ( c b ) 2 (a - b)^2 = (c - b)^2

( a b ) = ( c b ) 2 (a - b) = \sqrt(c - b)^2

( a b ) = ± ( c b ) (a - b) = \pm (c - b)

if taken as positive

( a b ) = + ( c b ) (a - b) = + (c - b)

a b = c b a - b = c - b

a = c b + b a = c - b + b

a = c a = c

and if taken as negative

( a b ) = ( c b ) (a - b) = - (c - b)

a b = c + b a - b = - c + b

a = c + b + b a = c + b + b

a = 2 b c a = 2b - c

so value of a a can be 2 b c 2b-c too. This makes the statement false.

The Real Jezzy C
May 27, 2018

LS (a-b) X (a-b) = (c-b) X (c-b) RS

a^2 + 2ab + b^2 = c^2 + 2cb + b^2

Subtract (b^2) and (c^2) from both sides and add (2ab) to both sides so that we get:

a^2 - 2ab + c^2 + b^2 - b^2 = c^2 - c^2 - 2cb + 2ab + b^2 - b^2

Once you cancel like terms you should end up with:

a^2 - c^2 = 2ab - 2cb

(a^2 - c^2) is a difference of squares and as such can be re written as:

(a+c) X (a-c) = 2b(a-c)

If we divide (a-c) from both sides we end up with:

a+c = 2b

If we wanted to isolate for either a or c we would end up with either (a = 2b - c) or (c = 2b - a). Since (c = 2b - a) =/= (c = a) the assertion is false.

Nick Blackshear
May 27, 2018

The equation c=2b-a makes this equation true with infinite solutions.

Let's find out if this is true or false.

If (a - b) × (a - b) = (c - b) x (c - b), then we have:

( a b ) 2 (a - b)^2 = ( c b ) 2 (c - b)^2 , so

a b |a - b| = c b |c - b|

Now we can divide the problem in 2 cases:

(a - b) = (c - b) or (a - b) = (b - c)

In the first case, we conclude that a = c, so the statement would be true.

In the second case, however, we conclude that a - 2b = c, which is only true when b = 0, so the statement is false.

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