Is It Obvious?

Algebra Level 2

True or False?

\quad If a , b , \color{#D61F06}{a},\color{#3D99F6}{b}, and c \color{#20A900}{c} are non-zero reals and a > b , \color{#D61F06}{a}>\color{#3D99F6}{b}, then a c > b c . \color{#D61F06}{a}^{\color{#20A900}{c}} > \color{#3D99F6}{b}^{\color{#20A900}{c}}.

False True

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

19 solutions

Andrew Ellinor
Oct 8, 2015

Assume that a a and b b are positive with a > b a > b .

If c > 0 c > 0 , then a c > b c a^c > b^c .

However, if c < 0 c < 0 , then a c > b c a^c > b^c is false!

In mathematics, if we say a statement is "true", then that means it is always true! But if there is even just one case where the statement is false, then the answer is false.

The statement is neither a tautology (always true) nor a contradiction (always false). In logic we would say that the statement is "satisfiable" because it is true for some non-zero real set of values for a, b and c. A lot of people seem to be implicitly adding a "For all" in front of the "non-zero real values..." bit, in which case the statement is indeed false. IMHO, as it is written, the statement cannot be assigned a truth value as it is not sufficiently well-defined. Which is what's leading to all of the disagreements (well that plus it's an Internet comments section)

Clyde Brown - 5 years, 8 months ago

Log in to reply

Would you agree that the 'then' is an implication, and since the implication does not exist, the conclusion made is false? So far as I can see, this is indisputable.

Jamie Eccleston - 5 years, 8 months ago

I think the original version probably did include a "For all" which has since gone missing. Or perhaps the answer choices had previously read "Always True" and "Always False".

(That and it's an Internet comments section!)

Andrew Ellinor - 5 years, 8 months ago

that's a very good deduction Brown!

Micheal Koomson - 5 years, 8 months ago

But for a theory ,we need soo many proofs to call it a theory... But if even one condition proves it wrong ,it ain't true.At least this is what i have been told by people

Ben Tennyson - 5 years, 7 months ago

but sir, the question says that non of the numbers, i.e. a,b, c are negative

Akash Pratap Singh - 5 years, 5 months ago

Nope, the statement implies that c is not negative. The statement of the problem is insufficient. OR there must be an option to answer "Depending on circumstances."

Roman Bykov - 5 years, 3 months ago
Nihar Mahajan
Oct 6, 2015

Simple counter example: a = 3 , b = 2 , c = 1 a=3 \ , \ b=2 \ , \ c=-1 . Thus ,

a c = 3 1 = 1 3 < 1 2 = 2 1 = b c a c < b c \large a^c = 3^{-1} =\dfrac{1}{3} < \dfrac{1}{2} = 2^{-1} = b^c \Rightarrow a^c<b^c

If a , b , c > 0 a,b,c>0 then it will be true

Department 8 - 5 years, 8 months ago

Log in to reply

I agree, Lakshya! But what if c < 0? The question is asking, "can you find choices for a, b, and c such that a > b, but a^c < b^c?" If you can, then the statement is false.

Andrew Ellinor - 5 years, 8 months ago

what if a,b,c are positive numbers. -_-

Ram Gautam - 5 years, 8 months ago

Log in to reply

what if a,b,c are negative numbers. -_- lol

Nihar Mahajan - 5 years, 8 months ago

The question is vague. The author didn't mention about c<0...If c>0, the answer is True.

D Kartikeyan - 5 years, 8 months ago

Log in to reply

The problem states veey clearly that a,b,c are non zero real numbers. Enough said.

Saurabh Singh - 5 years, 8 months ago

Log in to reply

That's a point!

Ben Tennyson - 5 years, 7 months ago

Hey there D. Even if the author doesn't mention it, the question is, "can you find choices for a, b, and c such that a > b, but a^c < b^c?" If you can, then the statement is false.

Andrew Ellinor - 5 years, 8 months ago

I agree. Answer can be either true or false. Question is incomplete.

Animesh Kumar - 5 years, 8 months ago

Log in to reply

In mathematics, if we say a statement is "true", then that means it is always true! But if there is even just one case where the statement is false, then the answer is false.

Nihar Mahajan - 5 years, 8 months ago

Log in to reply

@Nihar Mahajan Not only in mathematics, everywhere a statement stands "true" if it is true in all situations. Similarly a statement is "false" if it is false in all situations. Above question is true and false both if we take various examples. Third option should be "Cannot be determined with given data".

Animesh Kumar - 5 years, 8 months ago

Log in to reply

@Animesh Kumar Hey @Animesh Kumar . True in a mathematics context means "Always True", so here it would mean this statement is true for any choices of a, b, and c that you made. Are there choices for a, b, and c for which this statement is false? If there are, then you cannot mark it true!

Andrew Ellinor - 5 years, 8 months ago

Log in to reply

@Andrew Ellinor Hey Andrew, regret marking it "True". Please understand my concern and let me know your thoughts: 1. Had I marked it "False"; is the inequation false for all scenario? Answer is no. 2. There are just two options to choose from "True" or "False". How is the answer "False" if for some values of a,b and c it is "TRUE"

Animesh Kumar - 5 years, 8 months ago

Log in to reply

@Animesh Kumar In mathematics, "true" means "ALWAYS true". So if it's false for even just one choice of a, b, and c, then it cannot be marked as true. Here, try this one:

If p p is a prime number, then p p is an odd number. True or False?

Andrew Ellinor - 5 years, 8 months ago

Log in to reply

@Andrew Ellinor Without trying the question you have posed, want to restate what you just said in a little tweaked way. In mathematics, "false" means "ALWAYS false". So if it's true for even just one choice of a, b, and c, then it cannot be marked as false. Mathematics is not an alien subject where we have to blindly go by any theorem. What's correct for a "TRUE" shall hold correct for a "FALSE" too. Let me know if we differ.

Animesh Kumar - 5 years, 8 months ago

Log in to reply

@Animesh Kumar Seems reasonable, but what I'm saying is already encoded into the problem statement. I take it to mean, "For any choice of non-zero numbers a , b , a, b, and c c with a > b a > b , then a c > b c a^c > b^c ."

Andrew Ellinor - 5 years, 8 months ago

@Animesh Kumar Hi. No, a mathematical FALSE for an If-Then statement simply means NOT TRUE. (You are correct that TRUE for If-Then DOES mean ALWAYS TRUE.)

Therefore, for If-Then, TRUE if ALWAYS TRUE. FALSE if ANY condition exists where the If-Then is NOT TRUE.

Pamela Barnes - 5 years, 8 months ago
Niko Yochum
Oct 11, 2015

I think it is interesting that everyone is discussing the case where C < 0, but not where C>0 and a, b < 0. In this case, if C is even, a^c < b^c. Ex: a=-1, b=-3, c=2. a is greater than b, a,b, and c are non-zero reals, so the parameters of the question are satisfied, but a^c=1, and b^c=9, so b^c>a^c.

a,b given to be greater than 0

jatin khurana - 4 years, 11 months ago
Riaz Abdulla
Oct 11, 2015

A note to those who haven't taken math since high school. "TRUE sometimes" in mathematics is an automatic "FALSE". in math "TRUE" is automatically short for "TRUE in all cases"

If c c is negative then this inequality is False because a > b 1 a < 1 b a>b \implies \frac{1}{a}<\frac{1}{b}

Zoe Codrington
Sep 23, 2018

If it's negative it doesn't work. Negative... A^-1=1/A^1 If you got it wrong, it works with c=positive integers only and if a and b=Positive integer(unless c is even), It s sometimes true, not always

Mostakim Shakil
Feb 17, 2016

For all the Positive integer this statement , when a>b ; a 2 > b 2 a^{2} > b^{2} is true. but in case of negative number this Statement is false as we know every sqrt gives us two root values, one of which is negative. Like a = -2 and b= -3 , as always here a>b. But their squared value a 2 a^{2} = 4 and b 2 b^{2} =9 ,thus a 2 a^{2} < b 2 b^{2} .

Oli Hohman
Nov 29, 2015

True/False questions are the easiest to answer by attempting to produce a counter-example, i.e. a contradiction. If a = -1, b=-2, and c = 2, for example, you would have

(-1)^2>(-2)^2 1>4, which is a contradiction. Therefore, it's false.

Ashish Menon
Nov 28, 2015

The question clearly tells that a and b are non zero reals. So they can even be negative reals. let a and b be -1 & -2 respectively. Let c be -3. Then you will get why the answer is false

Joe Potillor
Oct 11, 2015

Answer is false, we can do this problem using proof by counter example. Since we're able to use non-zero real numbers, we can use any number that is not zero, for example (½) > (¼) if we let c = -1, the inequality becomes (½)^-1 > (¼)^-1 , which would be 2 > 4 which is never true in any situation.

Jamie Eccleston
Oct 10, 2015

The 'then' may be seen as a logical implication. Such an implication only exists if the subsequent statement (that a^c > b^c) is true whenever the preceding statement (the conjunction of the two criteria given in the question) is true. Because this is not the case, the implication does not exist. Whilst some may see this question as ambiguously written, it is simple to recognise that a^c < b^c can not be concluded from the criteria. Furthermore, for those who argue that the question needed to be specific about sign, it was: 'non-zero reals' incorporates all positive and negative real numbers, except zero.

Vinícius Almeida
Oct 10, 2015

There's no problems with the question at all. There's no need of an "always true" or "always false". If [conditions] then [conclusion]. If there is a conceivable scenario where the conditions are met yet there's another conclusion, then the statement is FALSE.

Bhavya Peshavaria
Oct 10, 2015

In one condition where a is 1 and b is -1 and c is any even number then a^c=b^c

Shiva Verma
Oct 8, 2015

The condition becomes false when c is a odd negative integer.

Razik Ridzuan
Oct 8, 2015

Counter example is one way to conclude the truth value of a statement..but doing in general where there are few cases (a,b>0 and c>0; a,b<0 and c>0; a,b>0 and c<0; a,b<0 and c<0; and so on...) certainly gives idea whether the statement is completely false or satisfiable for certain conditions..just to give a better perspective.. I do know that if we can give any counterexample to negate the statement, that would be the easiest way..

Albert Mourato
Oct 8, 2015

if a = -1, b = -2 and c = 1, then a>b. However if c = 2, b>a.

Govind Rathi
Oct 8, 2015

It depends in c whether the functions would be increasing or decreasing.Inequality won't hold for decreasing functions.

Chris White
Oct 8, 2015

This is trick question. Only limitation on C is it being non-zero, it could be anything else meaning there are an infinite possibilities where the answer is false... But if you assume C is a positive integer then you would get this wrong hence it being a trick question.

Hey, @Chris White . True in a mathematics context means "Always True", so here it would mean this statement is true for any choices of a, b, and c that you made. Are there choices for a, b, and c for which this statement is false? If there are, then you cannot mark it true!

Andrew Ellinor - 5 years, 8 months ago

Log in to reply

Yes, and I also go the answer correct. Just annoyed that over 50% of the questions I see on this site are trick questions that do not require knowledge of math at all and only a decent grasp of the English language.

Chris White - 5 years, 8 months ago

Log in to reply

"Over 50% of the questions I see on this site are trick questions that do not require knowledge of math at all". This is absolutely false and baseless idea.

Nihar Mahajan - 5 years, 8 months ago

Log in to reply

@Nihar Mahajan I would support Chris White. What he said is correct. You should have given another option C. Because the answer to this question is neither always true nor always false. Refer to Clyde Brown's comment : 'The statement is neither a tautology (always true) nor a contradiction (always false). In logic we would say that the statement is "satisfiable" because it is true for some non-zero real set of values for a, b and c'.

Samriddhi Singha Roy - 5 years, 8 months ago

If C is a positive even integer and "a" and "b" are the same numbers but negated (e.g. 1 and -1) then a^c = b^c and this is still false.

Jared Lerner - 5 years, 8 months ago
Woody Superman
Oct 8, 2015

Some example : 1) If a = 2, b = 1, c = -3 => a^c = 1/8 < 1 = b^c.

2) If a = -1, b = -2, c = 2 => a^c = 1 < 4 = b^c

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...