Floor Floor Floor Floor

Algebra Level 5

Given the equation

x x x x = 41 x \lfloor x \lfloor x \lfloor x \rfloor \rfloor \rfloor = 41

If α \alpha denote the largest negative value of x x which satisfy the equation above, while β \beta denote the smallest positive value of x x which satisfy the equation above.

What is the value of 17 α + 28 β 17\alpha + 28\beta ?


The answer is 41.

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.

1 solution

Christopher Boo
Mar 24, 2014

The approximate value of x x will be somewhere around

41 4 2.5304 \sqrt[4]{41}\approx2.5304

For β \beta ,

Let x = m n x=\frac{m}{n} , and m > n > 0 m>n>0 ,

m n k = 41 \frac{m}{n}k=41 where k k is the integer x x x \lfloor{x\lfloor{x\lfloor{x}}}\rfloor\rfloor\rfloor

k = 41 n m k=\frac{41n}{m}

m m must divide 41 41 for an integer k k , for the smallest positive value, m = 41 m=41

n n must be as large as possible, but bear in mind of the approximate value of x x . By trial and error, n = 14 n=14 .

Hence, β = 41 14 \beta=\frac{41}{14}

For α \alpha ,

The procedure is similar with the one above, only n = 17 n=-17 .

Hence, α = 41 17 \alpha=-\frac{41}{17}

17 α + 28 β = 41 17\alpha+28\beta=41


The main idea of the problem is by trial and error . However, next time if you see these kind of problem, it is the best for you to try n = 14 n=14 for β \beta , since the problem wants you to get 28 β 28\beta , clearly is because it needs integer value. For α \alpha , it is also the same, substitute 17 17 into n n immediately.

I picked x = 14 x=14 because the we need to find 28 β 28\beta .Before that I had evaluated how the value of the expression behaves with the various values of x.But if it was not given 28 β 28\beta I wonder if there is any other way to arrive at the correct guess.....I mean we can be incredibly close but converging on the exact value 41 14 \frac{41}{14} looks difficult to me without that given \2 8 β \28\beta .

Eddie The Head - 7 years, 2 months ago

Log in to reply

True, the 28 β 28\beta is really a big hint!

Christopher Boo - 7 years, 2 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...