Powers of 2 and 3

Algebra Level 1

Given that 2 x = 9 2^{x} = 9 and 2 7 y = 2 27^{y} = 2 find the value of 3 x y 3xy .


The answer is 2.

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3 solutions

Charlton Teo
Apr 3, 2016

Since we know 2 x = 9 2^{x} = 9 , we can rewrite that as log 2 9 = x \log_{2}9 = x . Also, since 2 7 y = 2 27^{y} = 2 we know 3 3 y = 2 3^{3y} = 2 which can be rewrite as log 3 2 = 3 y \log_{3}2 = 3y . Now, we see that we should convert the first equation to base three, which gives us log 3 9 log 3 2 \frac{\log_{3}9}{\log_{3}2} By multiplying 3 y 3y and x x , we actually multiply log 3 2 \log_{3}2 and log 3 9 log 3 2 \frac{\log_{3}9}{\log_{3}2} , leaving us with log 3 9 = 2 \log_{3}9 = 2

Great solution keep up the good work!

But here's a simpler method, instead of using logarithms, we can do this:

3 3 y = 2 3 3 x y = 2 x but 2 x = 9 3 3 x y = 9 = 3 2 3 x y = 2 \begin{aligned} 3^{3y}&=2\\ 3^{3xy}&=2^{x}\\ \text{but }2^x&=9\\ \therefore 3^{3xy}&=9=3^2\\ \implies3xy&=2 \end{aligned}

Sravanth C. - 5 years, 2 months ago

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Good work !

Aditya Sky - 5 years, 2 months ago

good sravanth

Arun Garg - 5 years, 2 months ago

Did the same way :D

Manish Mayank - 5 years, 2 months ago

I did it in the same way but I feel like the way you explained it is much better 😂😂😂

Kawaguchi Soushiro - 5 years, 1 month ago

learn from sravanth teo

Arun Garg - 5 years, 2 months ago

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I'm not criticizing his solution, I just pointed out a simpler method. Please don't take it otherwise ;)

Sravanth C. - 5 years, 2 months ago
Nandan Amish
Apr 20, 2016

taking logrithm on both sides for both eqns.. we get x= (2log3)/log2 and y=log2/(3log3) multiplying x and y we get x*y= 2/3 therefore 3xy= 3x2/3 = 2 answer...

2^x=9

2^x=3^2

2=3^(2/x)

27=2

3^3y=2

3^3y=3^(2/x)

3y=2/x

3yx=2

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