f ( x ) = ∣ ln ( ∣ x ∣ ) ∣ − 2 and g ( x ) = sin ( x + 2 3 π ) + 0 . 0 9 . Then find the total number of solutions of the equation f ( x ) − g ( x ) = 0
If
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Other than knowing that 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 ) , there would be two intersections of the graphs as long as f ( x ) < 1 . 0 9 , the maximum of g ( x ) (intuitively, trig peaks at one).
So I then set f ( x ) = m a x ( g ( x ) ) and simplified a bit for ∣ l n ( ∣ x ∣ ) ∣ = 3 . 0 9 . I'm currently only interested in what happens to the right, so from ± e ± 3 . 0 9 I used the value e 3 . 0 9 . This is the x -value where f ( x ) exceeds the maximum of g ( x ) .
Next, I find 2 ⌊ 2 π π + e 3 . 0 9 ⌋ for the number of peaks that g ( x ) reaches before this x -value, giving us 6 . However, I noticed here that a final two may exist that escape the reach of the max of g ( x ) while still intersecting it.
To address this, I found the derivatives of both sides and set them equal to each other, giving me x s i g n ( l n ∣ x ∣ ) = sin x . I did this because I knew that the derivative of f ( x ) would need to be less than the derivative of g ( x ) , then greater than the derivative of g ( x ) in order to intersect under g ( x ) , then escape before the max value. Here, I would have to sort of cheat and have my calculator solve this for values between 2 1 3 π and 2 1 5 π , which it does. I take this x-value and enter it for f ( x ) − g ( x ) , 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 intersections on the right side, so I know that I will have 1 6 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.
Without graphing only four solutions are seen
Nice graph player i many time obserbed his solution by graph nice
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The surest way to do this is by plotting the graphs of f ( x ) and g ( x ) and check the solutions. Using Newton's method it is found that there are 1 6 solutions as follows:
\[\begin{array} {} \pm 0.319613428 & \pm 5.17488463 & \pm 7.820756856 \\ \pm 10.71050396 & \pm 14.78521 & \pm 16.48571323 \\ \pm 21.91736565 & \pm 21.97375318 \end{array} \]