Solve the equation: $ 3^{2x+1}+4 \cdot 3^x = 15 $

algebra-precalculusexponential functionlogarithms

Solve the equation: $ 3^{2x+1}+4 \cdot 3^x = 15 $ where $x$ is a real number.

Background: Doing Olympiad question and got one from the book.

Attempt:

Let $3^x$ be $u$.

\begin{align*}
3^{2x+1} + 4 \cdot 3x – 15 &= 0 \\
3^{2x} \cdot 3^1 + 4 \cdot 3^x – 15 &= 0 \\
3^{2x} \cdot 3 + 4 \cdot 3^x – 15 &= 0 \\
3 \cdot 3^{2x} + 4 \cdot 3^x – 15 &= 0 \\
3u^2 + 4u – 15 &= 0
\end{align*}

Factoring the equation we get

$$ u=\frac{5}{3} $$

(We eliminate $u=-3$ as $x$ is real.)

$$ 3^x=\frac{5}{3} $$

Taking $\log$ both sides

$$ x \log 3 = \log\left(\frac{5}{3}\right) $$

Now I want to know how to further solve this. Also is there any easier way to solve this?

Best Answer

HINT:

You want to find value of $x$

It can be proceeded as

$x\log3=\log(\frac{5}{3})$

$x=\frac{\log(\frac{5}{3})}{\log3}$

$x=\log_3(\frac{5}{3})$ by converse of base change theorem

$x=\log_3 5-1$

To make your solution short by two or three steps take $\log_3()$ on both sides

$3^x=5/3$

$\log_3 3^x=\log_3 (5/3)$

$x=\log_3(5/3)$

$x=\frac{\log 5 - \log 3}{\log3}$

$x=\frac{0.698 - 0.477}{0.477}$

$x=\frac{0.221}{0.477}$ which can nearly be approximated to $0.5$