My, what a large number you are!

Algebra Level 2

m 2 = 2 3 × 3 4 × 5 6 \large m^ 2 = 2^3 \times 3^4 \times 5^6

If m m is positive, which of the following statements is true?

1000 < m < 2000 1000 < m < 2000 3000 < m < 4000 3000 < m < 4000 4000 < m < 5000 4000 < m < 5000 2000 < m < 3000 2000 < m < 3000

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9 solutions

Peter Macgregor
Jan 31, 2015

Surely the poser does not expect us to evaluate the RHS directly!

By grouping the powers appropriately write

m 2 = ( 10 × 9 × 5 ) 2 ( 10 × 5 ) = 45 0 2 × 50 m^2=({10}\times{9}\times{5})^2(10\times{5})=450^2\times50

and so

m = 450 × 50 m=450\times\sqrt{50}

and so

450 × 49 < m < 450 × 64 450\times\sqrt{49}<m<450\times\sqrt{64}

and it follows that

450 × 7 < m < 450 × 8 {450\times7}<{m}<450\times8

3150 < m < 3600 \implies 3150<m<3600

3000 < m < 4000 \implies 3000<m<4000

Nice way of using 7 < 50 < 8 7 < \sqrt{50} < 8 , so that you don't need to know what 2 \sqrt{2} is.

Chung Kevin - 6 years, 4 months ago

Um can someone please explain this to me? I thought all we needed to do was this 2^3=8 3^4=81 5^6=15625 15625+81+8=15714 and then square root 15714 to get your answer. this way its 15625+81+8=M^2 and then square root it and you get M=125.3

Karina Hoecker - 5 years, 7 months ago

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If you had a calculator, then yes that is a possible approach. The idea that I was going for, was how to solve this problem without external calculation aids.

Chung Kevin - 5 years, 6 months ago

You're going along the right lines but you're meant to be multiplying all of those powers, not adding them, which might make this a little time-consuming without a calculator.

Rachel Jones - 5 years, 5 months ago

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I actually did evaluate the whole right side in my head without calculator or paper. It took me 3 minutes though. I got 10 125 000 which is between $3000^2$ and $4000^2$ . Btw it is good excersise for your brain to do hard calculations in your head. It takes time to learn all the tricks but it's worth it.

Karol Gulaszek - 5 years, 2 months ago

Well um, i don't understand how do u group it..

Nathaniel Andika - 5 years, 6 months ago

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Simply multiply some of the terms together: (10 * 9 * 5)^2 => (2 5 * 3 3 * 5)^2 ; (10 * 5) => (2*5 * 5) ; Thus, this "grouping" of the terms really simplifies it more than how it is originally laid out. [(10 * 9 * 5)^2] * (10 * 5) is really the same thing as (2^3) * (3^4) * (5^6).

Navid Ghatri - 5 years, 5 months ago
Nihar Mahajan
Jan 30, 2015

m 2 = 2 3 × 3 4 × 5 6 \huge m^2 = 2^3 \times 3^4 \times 5^6

m = 8 × 3 2 × 5 3 \huge m = \sqrt{8} \times 3^2 \times 5^3

m = 2 × 2 × 9 × 125 \huge m = 2 \times \sqrt{2} \times 9 \times 125

m = 2250 × 2 \huge m = 2250 \times\sqrt{2}

m = 2250 × ( 1.414 ) \huge m = 2250 \times (1.414)

m = 3181.5 \huge m= 3181.5

S o , 3000 < m < 4000 : ) \huge So , \boxed{3000 < m < 4000} :)

i did this in sme way

Sushil Kumar - 5 years, 10 months ago

You are supposed to "Estimate" and not "Find out " the value of "M"

Krishna Ar - 6 years, 4 months ago

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In this problem, the computational complexity of finding the exact solution is the same as estimating it.

Janardhanan Sivaramakrishnan - 6 years, 4 months ago

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I disagree. It is much easier to estimate then it is to find the exact value. There are several solutions (and comments) which illustrate as such.

Chung Kevin - 6 years, 4 months ago

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@Chung Kevin In the brilliance of Nihar Mahajan estimating is waste of time when solving can be made easy.

J Chaturvedi - 6 years, 4 months ago

Not at all. Look at this- The above expression equals - 40 81 3125 = 3200 3125 40*81*3125= 3200*3125 ...thus its just a matter of few seconds to get it from here.

Krishna Ar - 6 years, 4 months ago

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@Krishna Ar Getting to the result in this manner required that you either 'knew' that 5 5 = 3125 5^5=3125 or had to do the computation.

Janardhanan Sivaramakrishnan - 6 years, 4 months ago

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@Janardhanan Sivaramakrishnan U need not know 5^5 Or 5^6 I just divided power by 2 and everyone know what 5^3 is. So calculations are better way and because the range is huge so i used 2^(1/2) as 1.5 Instead of 1.414 or whatever

prashant singh - 6 years, 4 months ago

@Krishna Ar It was actually simple ......but the problem is while answering it on my smartphone ......it sensed a touch while scrolling

ashutosh mahapatra - 6 years, 4 months ago

m = 3181.5 is not the actual value of 'm' . It is estimated , because 2^(1/2) is not actually equal to 1.414 .

Nihar Mahajan - 6 years, 4 months ago

could someone explain why doing m = 2^3/2 * 3^2 * 5^3 is not correct? I thought that sqrt(2^3) = 2^3/2 nevermind it turns out google though 2^3/2 was 2^3 / 2 doh!

Andrew Jones - 5 years, 7 months ago

Or -3181.5....

Chris Poulson - 4 years, 8 months ago

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But m is positive.

Chung Kevin - 4 years, 8 months ago
Curtis Clement
Jan 31, 2015

2 2 × 3 4 × 5 6 < m 2 < 2 4 × 3 4 × 5 6 2^2 \times\ 3^4 \times\ 5^6 < m^2 < 2^4 \times\ 3^4 \times\ 5^6 \Rightarrow 2 × 3 2 × 5 3 < m < 2 2 × 3 2 × 5 3 2 \times\ 3^2 \times\ 5^3 < m < 2^2 \times\ 3^2 \times\ 5^3 \Rightarrow 2250 < m < 4500 2250 < m < 4500 . Now m = 2250 2 3000 < m < 4000 m = 2250\sqrt{2}\therefore\ 3000< m < 4000

I felt that people would know that 2 1.4 \sqrt{2} \approx 1.4 , and thus that 1.4 < 2 < 1.5 1.4 < \sqrt{2} < 1.5 , which would make 3150 < m < 3375 3150 < m < 3375 , and hence reach the answer. Of course, since I chose a wider range, there are many other ways to approximate it.

Chung Kevin - 6 years, 4 months ago
Seth-Riley Adams
Apr 15, 2016

m 2 = 2 3 × 3 4 × 5 6 m 2 = 10125000 m = 1 0125000 m = 2250 2 2250 2 3182 3000 < m < 4000 m^2 = 2^3 \times 3^4 \times 5^6 \\ m^2 = 10125000\\ m = \sqrt10125000\\ m = 2250\sqrt2\\ 2250\sqrt2 \approx 3182 \\ \therefore 3000 < m < 4000

Antonio Rangel
Aug 11, 2015

Lucas Tell Marchi
Jan 31, 2015

Taking the square root

m = 2 × 9 × 125 × 2 = ( 4 + 4 + 1 ) × 250 2 = 2250 2 m = 2 \times 9 \times 125 \times \sqrt{2} = (4 + 4 + 1) \times 250 \sqrt{2} = 2250\sqrt{2}

Now, as 1 < 2 < 1.5 2000 < 2250 < 2250 2 = m < 3375 < 4000 1 < \sqrt{2} < 1.5 \;\;\; \Rightarrow \;\;\; 2000 < 2250 < 2250\sqrt{2} = m < 3375 < 4000 .

The lower bound has to be slightly tighter, so that you know that m > 3000 m > 3000 . Otherwise, you don't know if 2000 < m < 3000 2000 < m < 3000 which is one of the options.

Chung Kevin - 6 years, 4 months ago
Sadasiva Panicker
Nov 30, 2015

m^2 = 2^3x 3^4x5^6 = 10125000. Root of 19125000 approximately more than 3000. So m lies between3000 & 4000.

Syed Baqir
Jun 24, 2015

Easy way is to find (4000) ^2 and (3000)^2 Therefore, we can surely say that m must be between 3000 and 4000 as other inequalities do not satisfy the equation

Lu Chee Ket
Jan 30, 2015

Sqrt (10125000) = 3181.9805 {Correct to 4 decimal places.}

Yes, that's valid too :)

Chung Kevin - 6 years, 4 months ago

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Using calculator...

Paulo Carlos - 6 years, 4 months ago

I dont know about other languages but sqrt is a function in python . Do u know python ?? Or its just coincidence

prashant singh - 6 years, 4 months ago

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sqrt means square root. it's often used in many programming language

Hendro Yohanes - 6 years, 4 months ago

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