Any idea how to solve this equation? $x^2\log_{3}x^2-(2x^2+3)\log_{9}(2x+3)=3\log_{3}\frac{x}{2x+3}$
Any idea how to solve this equation?
-
0Sorry, I had no idea about this option. – 2011-12-01
2 Answers
Simplifying this equation by the step below: $(2x^2+3)\log_{9}(2x+3)=(2x^2+3)\frac{\log_{3}(2x+3)}{\log_{3}9}=\frac{2x^2+3}{2}\log_{3}(2x+3)$
Now the equation is : $x^2\log_{3}x^2-\frac{2x^2+3}{2}\log_{3}(2x+3)=3\log_{3}\frac{x}{2x+3}$
Eliminate the log symbol, we get: $\frac{x^{2x^2}}{(2x+3)^{\frac{2x+3}{2}}}=(\frac{x}{2x+3})^3$
Reduce and simplify: $x^{2x^2-3}=(2x+3)^{\frac{2x^2-3}{2}}$
Now, we see $2x^2-3=0$ or $x=(2x+3)^{\frac{1}{2}}$, namely, $x^2-2x-3=0$
So the roots are $x_1=\sqrt{\frac{3}{2}}, x_2=-\sqrt{\frac{3}{2}},x_3=-1,x_4=3$
Note that log function requires positive values, so $x_2$ and $x_3$should be discarded by substituting them into the original equation.
Conclusion: the roots are $x_1=\sqrt{\frac{3}{2}}, x_2=3$
Note that $\log_9(x) = \log_3(x)/\log_3(9) = \frac{1}{2}\log_3(x)$.
We need $2x+3\gt 0$, and hence $x\gt 0$, so that all logarithms can be evaluated.
So we can simplify the equation using the basic properties of the logarithms
$\begin{align*} x^2\log_{3}x^2-(2x^2+3)\log_{9}(2x+3) &= 3\log_{3}\left(\frac{x}{2x+3}\right)\\ 2x^2\log_3(x) - \frac{2x^2+3}{2}\log_3(2x+3) &= 3\log_3(x) - 3\log_3(2x+3)\\ (2x^2-3)\log_3(x) &= \left(\frac{2x^2+3}{2} - 3\right)\log_3(2x+3)\\ (2x^2-3)\log_3(x) &= \frac{2x^2-3}{2}\log_3(2x+3). \end{align*}$ From this, it is clear that any solutions to $2x^2-3=0$ will satisfy the equation. The solutions are $x=\sqrt{3/2}$ and $x=-\sqrt{3/2}$; but the latter cannot be used in the original equation. So one possibility is $x = \sqrt{\frac{3}{2}}.$
If $2x^2-3\neq 0$, then cancelling we get: $\begin{align*} (2x^2-3)\log_3(x) &= \frac{2x^2-3}{2}\log_3(2x+3)\\ \log_3(x) &= \frac{1}{2}\log_3(2x+3)\\ 2\log_3(x) - \log_3(2x+3) &= 0\\ \log_3\left(\frac{x^2}{2x+3}\right) &= 0\\ \frac{x^2}{2x+3} &= 1\\ x^2 -2x - 3 &= 0\\ (x-3)(x+1) &=0. \end{align*}$ Again, $x=-1$ cannot be used. So the only other solution is $x=3$.
So the solutions are $x=3$ and $x=\sqrt{\frac{3}{2}}$.