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Algebra Level 5

If f ( x ) = ln ( x ) 2 f(x)=|\ln(|x|)|-2 and g ( x ) = sin ( x + 3 π 2 ) + 0.09 g(x)=\sin(x+\frac{3\pi}{2})+0.09 . Then find the total number of solutions of the equation f ( x ) g ( x ) = 0 f(x)-g(x)=0


The answer is 16.

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1 solution

Chew-Seong Cheong
Apr 27, 2015

The surest way to do this is by plotting the graphs of f ( x ) f(x) and g ( x ) g(x) and check the solutions. Using Newton's method it is found that there are 16 \boxed{16} solutions as follows:

\[\begin{array} {} \pm 0.319613428 & \pm 5.17488463 & \pm 7.820756856 \\ \pm 10.71050396 & \pm 14.78521 & \pm 16.48571323 \\ \pm 21.91736565 & \pm 21.97375318 \end{array} \]

Moderator note:

Other than knowing that h ( x ) = f ( x ) g ( x ) h(x) = f(x) - g(x) is an even function. I'm curious to know whether it's possible to solve this without graphing.

Actually, I tried this problem without graphing and I did get a solution. I still used a calculator for a few values.

First, I noted the symmetry of the problem. Then, I went about solving for the number of intersections on the right side. I noticed that for every peak of g ( x ) g\left( x \right) , there would be two intersections of the graphs as long as f ( x ) < 1.09 f\left( x \right) <1.09 , the maximum of g ( x ) g\left( x \right) (intuitively, trig peaks at one).

So I then set f ( x ) = m a x ( g ( x ) ) f\left( x \right)=max\left( g\left( x \right)\right) and simplified a bit for l n ( x ) = 3.09 \left| ln\left( \left| x \right| \right) \right| =3.09 . I'm currently only interested in what happens to the right, so from ± e ± 3.09 \pm { e }^{ \pm 3.09 } I used the value e 3.09 { e }^{ 3.09 } . This is the x x -value where f ( x ) f\left( x \right) exceeds the maximum of g ( x ) g\left( x \right) .

Next, I find 2 π + e 3.09 2 π 2\left\lfloor \frac { \pi +{ e }^{ 3.09 } }{ 2\pi } \right\rfloor for the number of peaks that g ( x ) g\left( x \right) reaches before this x x -value, giving us 6 6 . However, I noticed here that a final two may exist that escape the reach of the max of g ( x ) g\left( x \right) while still intersecting it.

To address this, I found the derivatives of both sides and set them equal to each other, giving me s i g n ( l n x ) x = sin x \frac { sign\left( ln\left| x \right| \right) }{ x } =\sin { x } . I did this because I knew that the derivative of f ( x ) f\left( x \right) would need to be less than the derivative of g ( x ) g\left( x \right) , then greater than the derivative of g ( x ) g\left( x \right) in order to intersect under g ( x ) g\left( x \right) , then escape before the max value. Here, I would have to sort of cheat and have my calculator solve this for values between 13 π 2 \frac { 13\pi }{ 2 } and 15 π 2 \frac { 15\pi }{ 2 } , which it does. I take this x-value and enter it for f ( x ) g ( x ) f\left( x \right)-g\left( x \right) , and get an astoundingly positive nonzero number, so I know there are two more intersections on the right than I previously thought.

I end up with 8 8 intersections on the right side, so I know that I will have 16 16 intersections total.

So really, the solve thing is a bit iffy, but it seems you can get pretty far without hopping to the graphing option.

Austin Antonacci - 5 years, 7 months ago

Without graphing only four solutions are seen

Ram Sita - 3 years, 10 months ago

Nice graph player i many time obserbed his solution by graph nice

Ram Sita - 3 years, 10 months ago

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