MHB Find the slope of the secant to the curve f(x)=-3logx+2 between these points:

AI Thread Summary
The discussion focuses on finding the slope of the secant line for the function f(x) = -3log(x) + 2 between specified points. Participants confirm the calculations for the slopes using the formula m = (Y2 - Y1) / (X2 - X1), with results of approximately -0.903 and -1.056 for different intervals. The method of using derivatives to find the slope is also validated, with the derivative calculated as f'(x) = -3/(ln(10) * x). A suggestion is made to derive a general formula for the slope to simplify calculations for multiple intervals. Overall, the approach and calculations presented are affirmed as correct.
Vela1
Messages
2
Reaction score
0
The question was too long to post in the title so I just wrote down the first part. I hope this is alright. Here is the question that I am doing right now:

6beccca94bb60e9bef8dc95118c2936f.png


This is the graphical representation (thanks to Desmos Graphing Calculator):

34d481e2a7da1543a815be27e3c5b703.png


So I have substituted the points in the equation to get their respective y-values.
For example:

f(1.1) = -3log(1.1)+2
f(1.1) =~ 1.87

I've done the questions myself using this method to find the slope of the secant line, and I wanted confirmation that I was doing it right.

----------------------------------

I've written it on paper and unfortunately I don't have a scanner so I will just type out (i) and (ii) to show my rationale.

4.a)

i] m = (Y2-Y1) / (X2-X1)
= (1.097-2) / (2-1)
= -0.903 / 1
m = -0.903

ii] m = (Y2-Y1) / (X2-X1)
= (1.472-2) / (1.5-1)
= -0.528 / 0.5
m = -1.056

------------------------------------------------

I just wanted to know if I was doing it correctly, and if not how can I answer this question? Thanks in advance.
 
Mathematics news on Phys.org
Your example shows you're using $\log(x)=\log_{10}(x)$. If so, it appears your first one is correct; I didn't bother to check the second, as it's analogous. Looks fine to me!
 
OK, thank you. I just have one more question:

ec9f656f7a6a93b490aa2642ed779350.png


This is the second part to question #4. I wanted to do it myself before I confirmed my answer here.

Here are my steps:

I found the derivative of the initial equation: f(x) = -3log(x)+2

The derivative was: f(x) = (-3) / (ln[10] x (x))

So I substituted 1 for x, and the answer I got was -1.303. I rounded it to 3 decimal places. Is this the correct method, or is there another way?

Thanks in advance.
 
Yes, you are doing fine. I think what I would do is recognize all four questions are the same except for one parameter, $\Delta x$. So I would derive a formula into which I could then just plug into. In other words, work the problem once instead of four times.

The slope is given by:

$$m=\frac{\Delta f}{\Delta x}=\frac{f\left(x+\Delta x \right)-f(x)}{\Delta x}$$

Now, using the given definition of $f(x)$, we may write:

$$m=\frac{\left(-3\log\left(x+\Delta x \right)+2 \right)-\left(-3\log\left(x \right)+2 \right)}{\Delta x}=\frac{3\log\left(\frac{x}{x+\Delta x} \right)}{\Delta x}$$

With $x=1$, there results:

$$m=\frac{3\log\left(\frac{1}{1+\Delta x} \right)}{\Delta x}$$

a) $$\Delta x=1$$

$$m=\frac{3\log\left(\frac{1}{1+1} \right)}{1}=-3\log(2)\approx-0.903$$

b) $$\Delta x=\frac{1}{2}$$

$$m=\frac{3\log\left(\frac{1}{1+\frac{1}{2}} \right)}{\frac{1}{2}}=6\log\left(\frac{2}{3} \right)\approx-1.057$$

Now just plug-n-chug for the remaining two. :D

The formula we derived will be very useful in answering part b). Let $\Delta x$ get smaller and smaller until two successive results agree to 3 decimal places. Or, as you did, you can use calculus, but I suspect you are not expected to be able to differentiate.
 
Suppose ,instead of the usual x,y coordinate system with an I basis vector along the x -axis and a corresponding j basis vector along the y-axis we instead have a different pair of basis vectors ,call them e and f along their respective axes. I have seen that this is an important subject in maths My question is what physical applications does such a model apply to? I am asking here because I have devoted quite a lot of time in the past to understanding convectors and the dual...
Fermat's Last Theorem has long been one of the most famous mathematical problems, and is now one of the most famous theorems. It simply states that the equation $$ a^n+b^n=c^n $$ has no solutions with positive integers if ##n>2.## It was named after Pierre de Fermat (1607-1665). The problem itself stems from the book Arithmetica by Diophantus of Alexandria. It gained popularity because Fermat noted in his copy "Cubum autem in duos cubos, aut quadratoquadratum in duos quadratoquadratos, et...
Insights auto threads is broken atm, so I'm manually creating these for new Insight articles. In Dirac’s Principles of Quantum Mechanics published in 1930 he introduced a “convenient notation” he referred to as a “delta function” which he treated as a continuum analog to the discrete Kronecker delta. The Kronecker delta is simply the indexed components of the identity operator in matrix algebra Source: https://www.physicsforums.com/insights/what-exactly-is-diracs-delta-function/ by...
Back
Top