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Tuesday, May 10, 2016

Compare which of the following is bigger: 101611301631 versus 201642

Compare which of the following is bigger:

101611301631 versus 201642

My solution:

I know there are multiple approaches by which we can use to tackle the problem at hand. But if it were up to me, I would first look for the prime factorization for each number involved in the problem, and I found:

1016=23127

2016=25327

3016=231329

With the help of Wolfram Alpha, I've found

101611301631>201642

The effort remains now to first assume 101611301631>201642 is true and proved it so.

That is,

(23127)11(231329)31>(25327)42

Simplify the above we get:

2331271129313312931>2210384742

1271113312931>22103393384742

1271113312931>284384742

\displaystyle \color{yellow}\bbox[5px,purple]{127^{11}\cdot 377^{31}\gt 6^{84}\cdot 7^{42}}*

Here comes the difficult part of the process, we have to use the calculator to do a series of checking to see if we can get a simple relation between 377 and 7^a for some positive integer a such that 377\gt 7^a.

Observe that:

7^2=49,\,7^3=343\implies 373\gt 7^3

Therefore 373^{14}\gt (7^3)^{14} which is 373^{14}\gt 7^{42}

We have made some good progress, as we have kicked out the 7^{42} in the RHS of the equation * and also reduced the power of 377 from 31 down to 31-14=17.

At this point, if we can prove

\displaystyle \color{yellow}\bbox[5px,green]{127^{11}\cdot 377^{17}\gt 6^{84}}

Then we can conclude \displaystyle 1016^{11}\cdot 3016^{31}\gt 2016^{42} is correct.

Again, we have to find a relation between 127\cdot 377 and 6^b for some positive integer b such that 127\cdot 377\gt 6^b.

Observe that

127\cdot 377=47879\gt 6^6=46656

Taking 14th power on both sides of the inequality we get:

(127\cdot 377)^{14}\gt (6^6)^{14}

\displaystyle \color{black}\bbox[5px,orange]{127^{14}\cdot 377^{14}\gt 6^{84}}

It's obvious that

\displaystyle \color{yellow}\bbox[5px,blue]{127^{11}\cdot 377^{17}\gt 127^{14}\cdot 377^{14} } is true, therefore, we've proved that

\displaystyle 1016^{11}\cdot 3016^{31} is bigger than \displaystyle 2016^{42}.

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