Logarithms

Algebra Level 2

Evaluate the following:

9 log 3 ( 4 ) \Large 9^{\log_{3}(4)}

Do not use a calculator -- not even a slide rule!


The answer is 16.

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41 solutions

9 log 3 ( 4 ) = 3 2 log 3 ( 4 ) = 3 log 3 ( 4 2 ) = 3 log 3 ( 16 ) = 16 9^{\log_{3}(4)} = 3^{2*\log_{3}(4)} = 3^{\log_{3}(4^{2})} = 3^{\log_{3}(16)} = 16 .

Note that, in general, if x = a log a ( b ) x = a^{\log_{a}(b)} then log a ( x ) = log a ( a log a ( b ) ) = log a ( b ) \log_{a}(x) = \log_{a}(a^{\log_{a}(b)}) = \log_{a}(b) .

Now since the log function is one-to-one, we can conclude that x = b x = b .

we can do it this way too; let log 3 4 \log_3 4 be x {x} then by definition of logarithms 3 x = 4 3^{x}=4 ,we need 9 x 9^{x} , hence 16

Mukund Ratan - 6 years, 8 months ago

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very amazingly nice!

Ronik Gandhi - 6 years, 1 month ago

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What do you studying? Because i am also 16 years.

Poovallal Vallal - 5 years, 11 months ago

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@Poovallal Vallal all branches in mathematics by myself way in learning applying my theories

King Abner Mabanglo - 5 years, 8 months ago

But, how did 3^{x} = 4?

Zeeshan Khan S - 5 years, 10 months ago

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By definition of logarithms, if log a(b) = c, a^c = b. There you have it. Introducing a variable x, such that log 3(4) = x, by the definition above, 3^x = 4

Owen Leong - 5 years, 10 months ago

Really Awesome!!!!!

Kishore Mogambo - 5 years, 6 months ago

I did the exact same thing and got it!

Ram Padmanabhan - 5 years, 8 months ago

good solution

mmr raju - 5 years, 2 months ago

ok but when i used this method , we cant find 3^x=4 with out calculture i am used that solution

Patience Patience - 5 years, 2 months ago

De donde sale el 4?

Jaime Maldonado - 5 years, 7 months ago

Wonderful solution!!,good observation

A Former Brilliant Member - 5 years, 8 months ago

brilliant....

Ashutosh Kaul - 6 years, 4 months ago

But if you square the 4 to get 16 wouldn't you square 3^x to get 9^x2???

LiAnn Chantel - 5 years, 10 months ago

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(3^x)^2 = 3^(x*2) = (3^2)^x = 9^x

Owen Leong - 5 years, 10 months ago

NICE SOLUTION

Mriganka Saha - 5 years, 8 months ago

by chance 9 + 3 + 4 equals 16 and 16 happens to be the answer.

Ryan Choate - 6 years ago

hey 9^Log 3^4 right??

but at last that was 3 Log 3^16

billy liangganata - 6 years, 9 months ago

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Without using LaTex the expression would be something like 9^(log3(4)), where "log3" means log base 3. If you plugged this expression into WolframAlpha you would get the answer 16 16 .

Brian Charlesworth - 6 years, 9 months ago

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What is La Tex and WolframAlpha?

Min Yue Yong - 6 years, 9 months ago

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@Min Yue Yong LaTex is the typesetting system used here on Brilliant,(and elsewhere); enter "LaTex" into the Search box to find out more about how to use it. WolframAlpha is a popular computational knowledge engine. I'll give you the link .

Brian Charlesworth - 6 years, 9 months ago

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@Brian Charlesworth You just need a piece of paper and a pencil, or perhaps only your mind...

Ruben Sanchez - 6 years, 9 months ago

@Brian Charlesworth I had no idea Tex still existed, I used it in my first full time job in 1985 and assumed it had been long superseded by word processors and other WYSIWYG software. It used to take hours for the computer to create readable text from the markup, I assume that has changed!

Bryoni Pidsley - 5 years, 4 months ago

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@Bryoni Pidsley I also recall how long the markup process took back then, but it's virtually instantaneous now. It's still the standard for the production of scientific writings.

Brian Charlesworth - 5 years, 4 months ago

I don't understand how you do the last one. Why 3^log3 (16) = 16?

María Fernández - 6 years, 9 months ago

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That step is explained in my note. If we let x = 3 log 3 ( 16 ) x = 3^{\log_{3}(16)} , then

log 3 ( x ) = log 3 ( 3 log 3 ( 16 ) ) = log 3 ( 16 ) \log_{3}(x) = \log_{3}(3^{\log_{3}(16)}) = \log_{3}(16) .

(Note that in general log a ( a y ) = y \log_{a}(a^{y}) = y for any a > 0 a \gt 0 .)

So we end up with log 3 ( x ) = log 3 ( 16 ) \log_{3}(x) = \log_{3}(16) , which, since the log function is one-to-one, implies that x = 3 log 3 ( 16 ) = 16 x = 3^{\log_{3}(16)} = 16 .

(Note that when I say that a function f ( x ) f(x) is one-to-one I mean that f ( x ) = f ( y ) f(x) = f(y) if and only if x = y x = y .)

Brian Charlesworth - 6 years, 9 months ago

By definition of logarithm to the base a, we have log x = log b hence x = b as the log fn is one to one

Khor Bean Hwa - 6 years, 8 months ago

I did it with the same method!

Zakir Husain - 1 year ago

That's the way I thought it too!

Ferran Espuña - 5 years, 9 months ago

­ 9 log 3 4 = x log 3 9 log 3 4 = log 3 x ( log 3 4 ) l o g 3 9 = log 3 x ( log 3 4 ) ( 2 ) = log 3 x 3 log 3 4 2 = 3 log 3 x 16 = x ­{ 9 }^{ \log _{ 3 }{ 4 } }=x\\ \log _{ 3 }{ { 9 }^{ \log _{ 3 }{ 4 } } } =\log _{ 3 }{ x } \\ (\log _{ 3 }{ 4 } )log_{ 3 }{ 9 }=\log _{ 3 }{ x } \\ (\log _{ 3 }{ 4 } )(2)=\log _{ 3 }{ x } \\ { 3 }^{ \log _{ 3 }{ { 4 }^{ 2 } } }={ 3 }^{ \log _{ 3 }{ x } }\\ 16=x

simple and feasible explanation.....nice!!

Chaglani Ghansham - 6 years, 8 months ago

good explanation

Owais Asif - 6 years, 8 months ago

Great solution, easy to follow.

wibi wibisono - 6 years ago

whoao u rock buddy ! (y) got it

Bilal Raza - 6 years, 8 months ago

I used the same method.

Shrinidhi Ghatpande - 6 years, 8 months ago

i like it this way ... nice one sir ...

Ronnie Quipit - 6 years, 7 months ago

Good solution! But why have you added the number 3 on the last step?? Is it wrong to write just log(3) 4^2 = log(3) x ?

Gabriela Lamira - 4 years, 10 months ago

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I guess there is nothing wrong with your approach. I only add the number three in the last step cause I thought it would be more easy to see the answer.

Francisco Rodríguez - 4 years, 10 months ago

Simple one

RAHUL M - 4 years, 6 months ago
Nishant Prabhu
Oct 16, 2014

By rules of logarithms,

x l o g a b x^{log_{a}b} can be written as b l o g a x b^{log_{a}x} That makes things much more simpler.

THATS IT buddy, you're the man

Filip Slezák - 5 years, 6 months ago

Much simpler explanation!

Gwen Julius - 4 years, 10 months ago

yeah.. you got it bro

mark kim neis - 4 years, 10 months ago
Joe DeAlmo
Aug 21, 2015

Let x = l o g 3 ( 4 ) x = log_{3}(4) . Then we know that 3 x = 4 3^{x} = 4 . We want 9 x 9^{x} . So we can do this: 9 l o g 3 ( 4 ) = 9 x = ( 3 2 ) x 9^{log_{3}(4)} = 9^{x} = (3^{2})^{x} ( 3 2 ) x = 3 2 x = 3 x × 3 x (3^{2})^{x} = 3^{2x} = 3^{x} \times 3^{x}
And since 3 x = 4 3^{x} = 4 , we're left with 4 × 4 = 16 4 \times 4 = 16 .

Moderator note:

Great! That is the correct usage of the rules of exponents.

best, the simplest

Filip Slezák - 5 years, 6 months ago

Simple solution, yet amazing!

Joshua Calapardo - 5 years, 6 months ago
Kirk Bienvenu
Jul 11, 2015

Firstly, take a moment to understand this proof:

If x = y log b a x = y^{\log_{b}a}

Then, by taking log on both sides, we get log b x = log b ( y log b a \log_{b}x = \log_{b}(y^{\log_{b}a} )

Using the identity log n ( m x ) = x log n m \log_{n}(m^x) = x\log_{n}m

log b x = ( log b a ) log b y \log_{b}x = ({\log_{b}a})\log_{b}y

log b x = ( log b y ) log b a \log_{b}x = (\log_{b}y){\log_{b}a}

log b x = log b ( a log b y \log_{b}x = \log_{b}(a^{\log_{b}y} )

Removing log, x = a log b y x = a^{\log_{b}y}

So therefore, combining the first and last statements, we get y log b a = a log b y y^{\log_{b}a} = a^{\log_{b}y}

Moving on to the problem at hand:

Given x = 9 log 3 ( 4 ) x = 9^{\log_{3}(4)}

Using the above identity, we get

x = 4 log 3 ( 9 ) x = 4^{\log_{3}(9)}

x = 4 2 x = 4^2

x = 16 x = 16

Excelente, voy a tomar nota de esto en mis apuntes destacados. I LIKE so much

Jaime Maldonado - 5 years, 7 months ago

I will take note of this in my affairs highlight

Jaime Maldonado - 5 years, 7 months ago

I'd like to meet and try some mathematical issues with you. e-mail: [email protected]

Jaime Maldonado - 5 years, 7 months ago
Orville Evardo
Oct 10, 2015

How did you put that paper in the box? Brilliant e-mail [email protected]

Jaime Maldonado - 5 years, 7 months ago
Mahmoud Sayed
Jun 12, 2015

Sohaib Khan
Sep 24, 2014

9 log 3 4 = ( 3 log 3 4 ) 2 = 4 2 = 16 { 9 }^{ \log _{ 3 }{ 4 } }=({ 3 }^{ \log _{ 3 }{ 4 } })^{ 2 }=4^{ 2 }=16

Daniel Magro
Aug 1, 2015

L e t x = log 3 4 S o 9 l o g 3 ( 4 ) = 9 x B u t ; l o g 3 4 3 x = 4 3 x = 4 ( S q u a r e b o t h s i d e s ) 3 2 x = 4 2 ( 3 2 ) x = 16 9 x = 16 9 l o g 3 ( 4 ) = 16 Let\quad x=\log _{ 3 }{ 4 } \\ \\ So\quad 9^{ log_{ 3 }(4) }=9^{ x }\\ \\ But;\quad log_{ 3 }4\Leftrightarrow 3^{ x }=4\\ \\ \Rightarrow \quad 3^{ x }=4\quad (Square\quad both\quad sides)\\ \\ \Rightarrow \quad 3^{ 2x }=4^{ 2 }\\ \\ \Rightarrow \quad { \left( { 3 }^{ 2 } \right) }^{ x }=16\\ \\ \Rightarrow \quad 9^{ x }=16\\ \\ \therefore \quad 9^{ log_{ 3 }(4) }=16

Anand Iyer
Jun 5, 2015

Let x = 9 log 3 4 x=9^{\log_{3}4} , we take logs on both sides to get log(x)=2log4, this implies that x = 16 x=16 .

Kamble Sanjay
Sep 24, 2014

there is a rule...I don't have explanation now 9^log3(4) = 4^log3(9) you can actually rotate or swap the numbers, value remains the same.

Taru Bhardwaj
Sep 14, 2014

Since a^logb c ( actually b is the base and a is input) = c^logb a( same as above) Therefore, the above statement can be rewritten as 4^log3 3^2=4^2=16

Joel Kang
Jun 5, 2016

First, we can say 9 = 3^2.

Then, 9 log 3 ( 4 ) = 3 2 log 3 ( 4 ) {9}^{\log_{3}{(4)}} = {3}^{2\log_{3}{(4)}}

Using the power property, 2 log 3 ( 4 ) = log 3 ( 16 ) {2\log_{3}{(4)}}={\log_{3}{(16)}}

Now, using the Logarithmic Rules of Exponents, a log a ( b ) = b {a}^{\log_{a}{(b)}} = b

Substituting all the values, we get 3 log 3 ( 16 ) = 16 {3}^{\log_{3}{(16)}} = 16

Finally, we can conclude the solution to be 16 \boxed{16} .

Moderator note:

Simple standard approach.

Does using Fermi's solution count? (i.e. I looked and guessed)

9 l o g 3 ( 4 ) 9^{log_{3}(4)} = t,

taking log to the base e both sides,we have,

[ l o g 3 ( 4 ) ] l o g 9 [log_{3}(4)]*log 9 = log (t).

[ l o g 3 ( 4 ) ] 2 l o g ( 3 ) [log_{3}(4)]*2log(3) =log (t)

l o g e ( 4 ) l o g e ( 3 ) \frac{log_{e}(4)}{log_{e}(3)} *2log(3)) = log (t)

2 l o g ( 4 ) 2*log(4) =log (t)

l o g ( 4 ) 2 log(4)^{2} =log (t)

log 16 =log (t)

hence.t=16,ans.

Miguel Cabrita
Mar 26, 2016

This is simpler imho:

9 log 3 ( 4 ) = ( 3 × 3 ) log 3 ( 4 ) = ( 3 log 3 ( 4 ) ) × ( 3 log 3 ( 4 ) ) = 4 × 4 = 16 \begin{aligned} 9^{\log_3(4)} &=& (3\times 3)^{\log_3(4)}\\ &=&(3^{\log_3(4)})\times (3^{\log_3(4)})\\ &=& 4\times 4 \\ &=& 16 \end{aligned}

Nicolò Canavese
Feb 22, 2016

Let be l o g 3 ( 4 ) = x log_{3} (4)=x , so 3 x = 4 3^{x}=4 and since 9 = 3 2 9=3^{2} it means that the result is the square of 4 so 16

Amed Lolo
Feb 21, 2016

P=9^log3 (4) take log for two sides for base 4 log4 (p)=log3 (4)×log4 (9)=log3 (9)=2 so p=4^2=16######

let A=9^log(3)4 let B=log(3)4

Then A=3^2B and 3^B=4 (by logarithm)

then A=16

Do not try to memorize formulas or theorems ; just do it yourself

Anthony Stewart
Jan 21, 2016

9 l o g 3 ( 4 ) = ( 3 2 ) l o g 3 ( 4 ) = ( 3 l o g 3 ( 4 ) ) 2 = 4 2 = 16 9^{log_3(4)} = (3^2)^{log_3(4)} = (3^{log_3(4)})^2 = 4^2 = 16

Caroline Lui
Nov 4, 2015

you can rewrite that as log(base)9(of)x = log(base)3(of)4. Then, log(base)3^2(of)x can also be written as log(base)3(of)x^(1/2). Once the two expression on either side of the equal sine share the same base, you can set 4 equal to the square root of x, and therefore x is 16.

Zs Lin
Oct 27, 2015

let>> x=9^log3(4)

log3(x)=log3(9^log3(4))= log3(4)* log3(9)=log3(4)*2= 2 log3(4) = log3(4^2)

so x=4^2= 16

Prasit Sarapee
Oct 21, 2015

9^{log3(4)}=4^log3(9)

= 4^ log3(3^2)
= 4^ 2log3(3)
= 4^ 2(1)
= 4^2 = 16


Anapat Doudou
Oct 20, 2015

By luck babieesssssss ssssss

Rahul Chakwate
Sep 17, 2015

m^log b(n)=n^log b(m) hence question=4^log_3(9)=4^2

Ahsan Azhar
Sep 16, 2015

Lets suppose .... x= 9^log3 (4)....... x= 3^2*log3 (4).... x = 3^log3(4^2).....x=3^log3(16)......taking both sides log3.... log3(x)=log3[3^log3(16)]....... log3(x)=log3(16)log3(3).... log3(3)=1.....so log3(x)=log3(16).... by cancelling log3 on both sides we get.... x=16....hence.... 9^log3 (4)=16

Sayeed Motaleb
Aug 18, 2015

Let 9^{log3 (4)} = x

Now, 3^2{log3 (4)} =x

Or,3{log3 (4^2)} =x

Now according to formula, we obtain, x= 4^2=16.

Log a(b) = log a^c(b^c) (proof at the bottom) Log 3(4) = log 9(16) 9^log_9(16) = 16

PROOF Log a^c(b^c) = log(b^c) / log(a^c) = c*log(b) /c*log(a) =log(b) / log(a) =log a(b)

Tommy Bear
Aug 16, 2015

X=9^(log₃4) log₃X=log₃4 log₃9=2 log₃4=log₃16 X=16

Sushil Kumar
Jul 26, 2015

9^log3(4)

=> 3^2.log3(4)

m. log a= log a^m

therefore

3 ^log3(4^2)

if f(x)^log f(x) (X) = X ........... here f(x) is base in log

therefore

3^log3(4^2)= (4^2)=16 ....Ans

Musheer Uzzaman
May 31, 2015

9^{log 3(4)}=3^2{log 3(4)}=3{log 3(4^2)}= 3{log 3(16)}= 16

Javed Talukdar
Jan 12, 2015

9^log3(4)=3^log3(16) now go to basic knowledge, log3(16) means the exponent of 3 that will make it 16. suppose this exponent is x. so now we get 3^log3(16)=3^x=16. Bingo.

Girish Bellary
Jan 3, 2015

Say , y=9^[ log 4 to base 3 ].

Applying logarithms to base 3 on both sides ,

log y to base 3 = [ log 9 to base 3 ] * [ log 4 to base 3]. = 2 [ log 4 to base 3]. = [ log 4^2 to base 3 ]. = [ log 16 to base 3 ].

Comparing the arguments on both sides, y = 16.

Hence , 9^ [ log 4 to base 3 ] = 16.

Daniel Gabrić
Nov 13, 2014

9^log3(4) = 4^log3(9) = 4^2 = 16

Yusuf All-Husaini
Oct 30, 2014

you could also turn 3^(2log(3)(4)) into (3^log(3)(4)) (3^log(3)(4)) which intern equals 4 4=16

Sambhav Poddar
Oct 29, 2014

Let log_{3}(4)=y

4 = 3^{y}

squaring both sides

16 = 9^{y}

Hence 9^{log_{3}(4)} = 16

Sadasiva M
Oct 7, 2014

9^(log3(4)) = (3^2)^(log3(4)) = 3^(log3(4^2)) = 3^(log3(16)) = 16

Pankaj Patil
Sep 27, 2014

9^(log3(4))=3^(2(log3(4)))=3^(log3(16))=16

Sai Raghava
Sep 26, 2014

9^(log3(4))=3^(2(log3(4)))=3^(log3(16))=16

Let X=given function.

than take log both side,

apply the basic principles of logarithms,

we get 16.

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