# Thread: Exponential simultaneous equations

1. ## Exponential simultaneous equations

A little help with these would be of help!

i) x^y= y^x
x^2=y^3

Solve for x and y.

ii)a^x=x^y
a^y=x^x

Same here.

2. Hello Arka

Welcome to Math Help Forum!
Originally Posted by Arka
A little help with these would be of help!

i) x^y= y^x
x^2=y^3

Solve for x and y.

ii)a^x=x^y
a^y=x^x

Same here.
By inspection, we can see that one solution is $\displaystyle x = y = 1$. But if $\displaystyle y \ne 1$, then we can take logs, knowing that $\displaystyle \log y \ne 0$:
$\displaystyle \left\{\begin {array}{l} x^y = y^x\\
x^2=y^3\end{array}$
$\displaystyle \Rightarrow \left\{\begin{array}{l} y\log x = x\log y\\
2\log x = 3 \log y \end{array}$

$\displaystyle \Rightarrow \dfrac{\log x}{\log y}=\dfrac xy=\dfrac32$, provided $\displaystyle \log y \ne 0$.

$\displaystyle \Rightarrow y = \tfrac23x$

$\displaystyle \Rightarrow y^3 = \tfrac{8}{27}x^3=x^2$

$\Rightarrow 8x^3 - 27x^2=0$

$\displaystyle \Rightarrow x = 0$ or $\displaystyle x = \tfrac{27}{8}$

$\displaystyle x=0$ clearly gives us problems with $\log x$. So we're left with the solution $\displaystyle x = \tfrac{27}{8},\;y = \tfrac94$.

I haven't looked at (ii). I think you could begin in a similar way.

3. ## LOG!!??

Cant we solve it without using logarithms? Coz in this sum , my prof said that we are not allowed to use log!

4. Hello Arka
Originally Posted by Arka
Cant we solve it without using logarithms? Coz in this sum , my prof said that we are not allowed to use log!
Yes, you can; although it would have been helpful if you had said so at the beginning.

I prefer to use logs, because it avoids handling fractional exponents, but here goes:
$y^3 = x^2$

$\Rightarrow y = x^{\frac23}$
Substitute into the equation $x^y = y^x$:
$x^{(x^{\frac23})}=\big(x^{\frac23\big)^x}$
$=x^{(\frac23x)}$
If we now compare the exponents on each side of the equation, we get:
$\Rightarrow x^{\frac23}=\frac23x$
Cube both sides:
$\Rightarrow x^2 = \frac{8}{27}x^3$
Then, as before, $x = ...$