Crazy Inequalities

Algebra Level 2

Given three numbers such that 0 < a < b < c < 1 0 < a < b < c < 1 , define

A = a a b b c c , B = a a b c c b , C = a b b c c a . A = a^{a}b^{b}c^{c}, \quad B = a^{a}b^{c}c^{b} , \quad C = a^{b}b^{c}c^{a}.

How do the values of A , B , C A, B, C compare to each other?

C < A < B B < C < A C < B < A A < C < B

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5 solutions

Daniel Liu
Apr 9, 2014

Let's first compare A A and B B . Let's first assume A > B A > B . This means a a b b c c > a a b c c b a^ab^bc^c > a^ab^cc^b . This simplifies to c c b > b c b c^{c-b} > b^{c-b} , which is clearly true. Therefore A > B A > B .

Now let's compare B B with C C . Again, assume that B > C B > C . This means that a a b c c b > a b b c c a a^ab^cc^b > a^bb^cc^a . This simplifies to c b a > a b a c^{b-a} > a^{b-a} , which is true. Therefore B > C B > C .

Thus, the answer is A > B > C A > B > C , or C < B < A \boxed{C < B < A} .

Consider the ratio A B \frac{A}{B} , which will exceed one if A > B A > B . Reducing the fraction, we find A B = b b c c b c c b = b b c c c b = ( c b ) c b . \frac{A}{B} = \frac{b^bc^c}{b^cc^b} = b^{b - c}c^{c - b} = \left(\frac{c}{b}\right)^{c - b}. But c > b c > b , so c b > 1 \frac{c}{b} > 1 . Since the exponent c b c - b is positive, we know A B = ( c b ) c b > 1 \frac{A}{B} = \left(\frac{c}{b}\right)^{c - b} > 1 , and A > B A > B . Likewise, the ratio B C \frac{B}{C} yields B C = ( c a ) b a > 1 , \frac{B}{C} = \left(\frac{c}{a}\right)^{b - a} > 1, so B > C B > C .

Zaber Mahbub
Jan 3, 2016

Just assume any three value of a,b,c such as 0.1,0.2 and 0.3. Now use calculator. You get A>B>C or C<B<A.

Wrong answer

STgm T - 11 months ago
Zoe Codrington
Sep 24, 2018

A is the biggest because it contains the biggest possible power when using only a, b or c of c^c.

C 'wastes' base c, a big base on a, a small power, and wastes b as power on a as base. But B has a^a to stop this. So C is smaller than b.

Jonathan Troville
Aug 21, 2016

The easiest method is to choose any real numbers on the interval. So, WLOG we can define a, b, and c to be rational.

Hence, let a=(1/8), b=(1/4), c=(1/2)

==> A = 1/(2^(1/8))^11

    B = 1/(2^(1/8))^13

    C = 1/(2^(1/8))^15

Taking ratios:

1) A/B = 2^(1/4)

2) A/C = 2^(1/2)

3) B/C = 2^(1/4)

We know that x^(1/2)>x^(1/4) for x > 1 .

Hence, 2^(1/2)>2^(1/4)

Also, x^(1/2) and x^(1/4) are > 1 for x >1

So, 2^(1/2)>2^(1/4)>1

==> A > C; A>B; B>C

Hence, A>B>C ==> C<B<A

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