Number of Real solution in x of eqn:a^x+b^x=c^x(0<a<b<c)
by adityaguharoy, Apr 12, 2017, 6:37 AM
Given real numbers
.
Solve
.
Solution
The more General Case

Solve

Solution
Divide both sides by
, then we get
The left side of the above equation is monotone decreasing while the right side of the equation is monotone increasing.
So, maximum number of solutions is
.
Further note that if we consider the original equation
and now divide both sides of this equation by
then we get,
Then, define 
Then ,
But when taking with
we get
and also
.
So, we can conclude that there exists some
such that 
(one elegant way to make this claim more rigorous is to use the definition of limit
)
So, by the Intermediate Value Property, we get that there is at least one real solution to the equation
This shows that the equation
has exactly one solution and more precisely this solution is a positive real number when
.


So, maximum number of solutions is

Further note that if we consider the original equation




Then ,




So, we can conclude that there exists some


(one elegant way to make this claim more rigorous is to use the definition of limit

So, by the Intermediate Value Property, we get that there is at least one real solution to the equation



The more General Case
In general we can devise a method of solving
when
by dividing both sides of this equation by
and then the similar argument gives a solution.


