February 21st, 2017, 12:32 AM  #1 
Member Joined: Feb 2017 From: East U.S. Posts: 40 Thanks: 0  Please teach me how to do this
The limit represents f '(c) for a function f(x) and a number c. Find f(x) and c. lim (Δx>0) [78(1+Δx)]1(1)/Δx So the story is... my teacher assigns this as a homework problem that's gonna turn up on our exam tomorrow, but without going over a single problem with a triangle thing in it. I looked it up and apparently it's a "change" symbol? Please, I really just need the steps written down on how to solve this so I can at least try to learn a little before my next exam in about 11 hours. I'm willing to try to learn, and not just looking for the answer, but I have no time... ANY advice would be great, thanks! 
February 21st, 2017, 01:49 AM  #2 
Senior Member Joined: Feb 2016 From: Australia Posts: 1,356 Thanks: 464 Math Focus: Yet to find out. 
The 'little triangle' is an uppercase delta in Greek. And yes, it usually means 'change in'. When you see $\Delta x$, you can say 'change in x'. Although this by itself doesn't mean much. I can't really make out what your function is supposed to be due to some peculiar bracketing. $\displaystyle \lim\limits_{\Delta x \rightarrow 0} \dfrac{7  8(1 + \Delta x)}{\Delta x}$ or, $\displaystyle \lim\limits_{\Delta x \rightarrow 0} 7  8(1 + \Delta x) +\dfrac{1}{\Delta x}$??? Or something else... 
February 21st, 2017, 02:46 AM  #3  
Member Joined: Feb 2017 From: East U.S. Posts: 40 Thanks: 0  Quote:
lim Δx>0 In the numerator of the function, I have "[78(1+Δx)](1)" (there are brackets for some reason) And in the denominator, I just have a "Δx" Hopefully this clarified everything. Last edited by nbg273; February 21st, 2017 at 03:35 AM.  
February 21st, 2017, 03:48 AM  #4 
Math Team Joined: Dec 2013 From: Colombia Posts: 6,973 Thanks: 2295 Math Focus: Mainly analysis and algebra 
$$f'(c)= \lim_{\Delta x \to 0} \frac{f(c+\Delta x)  f(c)}{\Delta x}$$ $f(c+\Delta x) = 78(1+\Delta x)$ and $f(c) = 1$. By inspection, you can then suggest a value for $c$ and an expression for $f(x)$. 
February 21st, 2017, 04:03 AM  #5 
Member Joined: Feb 2017 From: East U.S. Posts: 40 Thanks: 0  Sorry, I'm still confused on what f(x) is... I'm bad at math...

February 21st, 2017, 04:29 AM  #6 
Math Team Joined: Jan 2015 From: Alabama Posts: 2,731 Thanks: 707 
It looks like f(x) is intended to be f(x)= 7 8x. Then and . So . That is, for all nonzero , . So what is the limit as goes to 0? 
February 21st, 2017, 05:17 AM  #7 
Member Joined: Feb 2017 From: East U.S. Posts: 40 Thanks: 0  

Tags 
teach 
Thread Tools  
Display Modes  

Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
please add me and teach me using MyMathForum !!  MATHEMATICIAN  New Users  12  August 23rd, 2013 02:23 AM 
How to teach yourself...  MattJ81  New Users  7  July 29th, 2011 10:17 PM 
will someone teach me geometry?  cameron  Geometry  1  June 19th, 2009 05:35 AM 
please add me and teach me using MyMathForum !!  MATHEMATICIAN  Calculus  0  December 31st, 1969 04:00 PM 