Solve exponential equation $6^x + 4^x = 9^x$

Given the equation $6^x + 4^x = 9^x$, solve for all $x$.

Solution

The idea is to divide the whole equation by $4^x$:

$$\begin{array}{rrl} & 4^x + 6^x &= 9^x\\ \Leftrightarrow & 1 + \frac{6^x}{4^x} &= \frac{9^x}{4^x}\\ \Leftrightarrow& 1 + \left(\frac{3}{2}\right)^x &= \left(\left(\frac{3}{2}\right)^2\right)^x = \left(\left(\frac{3}{2}\right)^x\right)^2\\ \end{array}$$

In this way we get a quadratic formula in $q=\left(\frac{3}{2}\right)^x$:

$$\begin{array}{rrl} & 1 + q &= q^2\\ \Leftrightarrow &q^2 - q - 1 &= 0\\ \Leftrightarrow &(q-\frac{1}{2})^2 &= \frac{5}{4}\\ \Leftrightarrow &q &= \frac{1}{2}\pm\frac{\sqrt{5}}{2}\\ \Leftrightarrow &q &= \frac{1\pm \sqrt{5}}{2}\\ \end{array}$$

Since $q=\left(\frac{3}{2}\right)^x>0$, we can ignore the negative solution and proceed with the positive (golden ratio) $q = \frac{1+ \sqrt{5}}{2}$. Setting both variants for $q$ equal yields

$$\begin{array}{rrl} &\left(\frac{3}{2}\right)^x &= \frac{1+ \sqrt{5}}{2}\\ \Leftrightarrow &x\cdot \ln\left(\frac{3}{2}\right) &= \ln\frac{1+ \sqrt{5}}{2}\\ \Leftrightarrow &x &= \frac{\ln\frac{1+ \sqrt{5}}{2}}{\ln\left(\frac{3}{2}\right)} \\ &&= 1.1868143\dots \end{array}$$

Solution using Newton Raphson method

We can solve this problem also numerically using Newton Raphson method.

Let $f(x) = 4^x + 6^x - 9^x$, then $f'(x) = 4^x \ln 4 + 6^x \ln 6 - 9^x \ln 9$. Since we want the solution $x$, for which $f(x) = 0$, we try some small values: $f(1) = 1$, $f(2) = -29$. That means $f(x)=0$ must be somewhere between 1 and 2, so we start with $x_0=1.5$:

The next better estimate for $x$ is given by $x_{n+1} = x_n - \frac{f(x_n)}{f'(x_n)}$: