Can you solve this simple equation using algebra? I doubt it!

Algebra Level 1

What is the largest value of x x which satisfies

9 x = 3 x ? 9x=3^x?

It is neither a linear function nor an exponential function If you solve it, give us your solution, please.


The answer is 3.

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14 solutions

Isaiah Simeone
Sep 20, 2014

Graph the two functions 9x and 3^x and look for where the two intercept.

The intercept furthest right.

Feathery Studio - 5 years, 10 months ago
Lu Chee Ket
Aug 4, 2014

Do not doubt. For y = 9 x and y = 3^x,

At x = 0, 3^x > 9 x such that 1 > 0;

At x = 1, 9 x > 3^x such that 9 > 3;

At x = 2, 9 x > 3^x such that 18 > 9;

At x = 3, 3^x = 9 x such that 27 = 27;

At x = 4, 3^x > 9 x such that 81 > 36;

d y/ d x = 9 and d y / d x = (Ln 3) 3^x respectively where (Ln 3) 3^x > 9 at x >= 3+.

Largest value presumed real x only.

There are two intersect points, one at x = 0.127869 and another one at x = 3 only. Therefore, the largest value of x is 3.

Hassan Raza
Jul 30, 2014

9 x = 3 x . . . . . . . . . . ( A ) L e t " 3 " i s a n s w e r C h e c k i n g . P u t t i n g x = 3 i n ( A ) 9 ( 3 ) = 3 ( 3 ) = > 27 = 27 S a t i s f y 9x={ 3 }^{ x }..........(A)\\ Let\quad "3"\quad is\quad answer\\ Checking.\\ Putting\quad x=3\quad in\quad (A)\\ 9(3)={ 3 }^{ \left( 3 \right) }\\ =>\boxed { 27=27 } \\ Satisfy

Same way of thinking here

Sem David Sitanggang - 6 years, 10 months ago

Guess and check really. 2 and 1 don't work but 3 works because 9*3 is the same thing as 3^3

Jay Nagar
Jan 17, 2015

take log base both sides

Kritarth Lohomi
Jan 14, 2015

Easy just use log (3) 3is the base

Anoushka Agrawal
Jul 29, 2014

(9)(3)=27 (3)(3)(3)=27 so x=3 after 3 no other number can be the value of x

Mazen Melouk
Jul 26, 2014

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

Ayushi Gupta
Jul 22, 2014

9x3=3x3x3=27.so x=3

Upendro Meska
Jul 22, 2014

9×3=3^3 27=27

Sonali Srivastava
Jul 21, 2014

9*3=27 and 3^3=27 hence x=3

Kefan Xie
Jul 19, 2014

9 = 3^3; 9x = 3^3*x; so question is 3^2 * x = 3^x, assume x = 3^y, then question becomes 3^(2+y) = 3^(3^y) == 3^(2+y) = 3^(3y); 2 + y = 3y, y = 1, and follows x = 3.

The best answer so far! Just need to correct into: "9 = 3^2; 9x = 3^2*x; so question is 3^2 * x = 3^x, assume x = 3^y, then question becomes 3^(2+y) = 3^(3^y) ==> 3^(2+y) = 3^(3y) ==> 2 + y = 3y, y = 1, and follows x = 3."

Linkin Duck - 6 years, 10 months ago

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Excellent solution!!

Aran Pasupathy - 6 years, 4 months ago

9x= 3^2 *x why it becomes 3^(2+y) not (3^2)+y ? Can u explain this one?

Christian Lauw - 6 years, 10 months ago

9x3 = 3³--> 9+9+9=3x3x3 --> 27 = 27 --> Thus, in the Naturals numbers three is the single answer...

Not different from Gowthaman's solution. You've simply shown us that 3 3 works, and used trial & error. And this is not a proof that this is the only solution in the natural numbers, although it's quite clear that it is if you observe how quickly the value of the RHS in x = 3 x 2 x=3^{x-2} rises compared to the LHS.

mathh mathh - 6 years, 11 months ago
Gowthaman J
Jul 15, 2014

9*3=3^3=27

How can we know this is a unique solution? You haven't posted any solution btw, you've just showed us that 3 works, but the OP is searching for a solution that doesn't involve trial & error.

mathh mathh - 6 years, 11 months ago

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In fact, that is not the only solution. Another solution is around 0.13 0.13 .

Daniel Liu - 6 years, 11 months ago

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Well, I'm reporting this question then, since there are multiple ways of answering it.

mathh mathh - 6 years, 11 months ago

This question can be solved if you plot the graphs of y=9x and y=3^x separately and check for intersection points,one can find that both graphs intersect only at one possible point i.e. 3.

Mayank Mishra - 6 years, 10 months ago

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They also intersect at around 0.127869 0.127869 . These two are the only points they intersect at, though.

mathh mathh - 6 years, 10 months ago

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