# 1.5 Exponential and logarithmic functions  (Page 3/17)

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$A\left(t\right)=P{\left(1+\frac{r}{n}\right)}^{nt}.$

What happens as $n\to \infty ?$ To answer this question, we let $m=n\text{/}r$ and write

${\left(1+\frac{r}{n}\right)}^{nt}={\left(1+\frac{1}{m}\right)}^{mrt},$

and examine the behavior of ${\left(1+1\text{/}m\right)}^{m}$ as $m\to \infty ,$ using a table of values ( [link] ).

 $\mathbit{\text{m}}$ $10$ $100$ $1000$ $10,000$ $100,000$ $1,000,000$ ${\mathbf{\left(}\mathbf{1}\mathbf{+}\frac{\mathbf{1}}{\mathbit{\text{m}}}\mathbf{\right)}}^{\mathbit{\text{m}}}$ $2.5937$ $2.7048$ $2.71692$ $2.71815$ $2.718268$ $2.718280$

Looking at this table, it appears that ${\left(1+1\text{/}m\right)}^{m}$ is approaching a number between $2.7$ and $2.8$ as $m\to \infty .$ In fact, ${\left(1+1\text{/}m\right)}^{m}$ does approach some number as $m\to \infty .$ We call this number $e$    . To six decimal places of accuracy,

$e\approx 2.718282.$

The letter $e$ was first used to represent this number by the Swiss mathematician Leonhard Euler during the 1720s. Although Euler did not discover the number, he showed many important connections between $e$ and logarithmic functions. We still use the notation $e$ today to honor Euler’s work because it appears in many areas of mathematics and because we can use it in many practical applications.

Returning to our savings account example, we can conclude that if a person puts $P$ dollars in an account at an annual interest rate $r,$ compounded continuously, then $A\left(t\right)=P{e}^{rt}.$ This function may be familiar. Since functions involving base $e$ arise often in applications, we call the function $f\left(x\right)={e}^{x}$ the natural exponential function    . Not only is this function interesting because of the definition of the number $e,$ but also, as discussed next, its graph has an important property.

Since $e>1,$ we know ${e}^{x}$ is increasing on $\left(\text{−}\infty ,\infty \right).$ In [link] , we show a graph of $f\left(x\right)={e}^{x}$ along with a tangent line to the graph of at $x=0.$ We give a precise definition of tangent line in the next chapter; but, informally, we say a tangent line to a graph of $f$ at $x=a$ is a line that passes through the point $\left(a,f\left(a\right)\right)$ and has the same “slope” as $f$ at that point $.$ The function $f\left(x\right)={e}^{x}$ is the only exponential function ${b}^{x}$ with tangent line at $x=0$ that has a slope of 1. As we see later in the text, having this property makes the natural exponential function the most simple exponential function to use in many instances.

## Compounding interest

Suppose $\text{}500$ is invested in an account at an annual interest rate of $r=5.5%,$ compounded continuously.

1. Let $t$ denote the number of years after the initial investment and $A\left(t\right)$ denote the amount of money in the account at time $t.$ Find a formula for $A\left(t\right).$
2. Find the amount of money in the account after $10$ years and after $20$ years.
1. If $P$ dollars are invested in an account at an annual interest rate $r,$ compounded continuously, then $A\left(t\right)=P{e}^{rt}.$ Here $P=\text{}500$ and $r=0.055.$ Therefore, $A\left(t\right)=500{e}^{0.055t}.$
2. After $10$ years, the amount of money in the account is
$A\left(10\right)=500{e}^{0.055·10}=500{e}^{0.55}\approx \text{}866.63.$

After $20$ years, the amount of money in the account is
$A\left(20\right)=500{e}^{0.055·20}=500{e}^{1.1}\approx \text{}1,502.08.$

If $\text{}750$ is invested in an account at an annual interest rate of $4%,$ compounded continuously, find a formula for the amount of money in the account after $t$ years. Find the amount of money after $30$ years.

$A\left(t\right)=750{e}^{0.04t}.$ After $30$ years, there will be approximately $\text{}2,490.09.$

## Logarithmic functions

Using our understanding of exponential functions, we can discuss their inverses, which are the logarithmic functions. These come in handy when we need to consider any phenomenon that varies over a wide range of values, such as pH in chemistry or decibels in sound levels.

can you give me a problem for function. a trigonometric one
state and prove L hospital rule
I want to know about hospital rule
Faysal
If you tell me how can I Know about engineering math 1( sugh as any lecture or tutorial)
Faysal
I don't know either i am also new,first year college ,taking computer engineer,and.trying to advance learning
Amor
if you want some help on l hospital rule ask me
it's spelled hopital
Connor
hi
BERNANDINO
you are correct Connor Angeli, the L'Hospital was the old one but the modern way to say is L 'Hôpital.
Leo
I had no clue this was an online app
Connor
Total online shopping during the Christmas holidays has increased dramatically during the past 5 years. In 2012 (t=0), total online holiday sales were $42.3 billion, whereas in 2013 they were$48.1 billion. Find a linear function S that estimates the total online holiday sales in the year t . Interpret the slope of the graph of S . Use part a. to predict the year when online shopping during Christmas will reach \$60 billion?
what is the derivative of x= Arc sin (x)^1/2
y^2 = arcsin(x)
Pitior
x = sin (y^2)
Pitior
differentiate implicitly
Pitior
then solve for dy/dx
Pitior
thank you it was very helpful
morfling
questions solve y=sin x
Solve it for what?
Tim
you have to apply the function arcsin in both sides and you get arcsin y = acrsin (sin x) the the function arcsin and function sin cancel each other so the ecuation becomes arcsin y = x you can also write x= arcsin y
Ioana
what is the question ? what is the answer?
Suman
there is an equation that should be solve for x
Ioana
ok solve it
Suman
are you saying y is of sin(x) y=sin(x)/sin of both sides to solve for x... therefore y/sin =x
Tyron
or solve for sin(x) via the unit circle
Tyron
what is unit circle
Suman
a circle whose radius is 1.
Darnell
the unit circle is covered in pre cal...and or trigonometry. it is the multipcation table of upper level mathematics.
Tyron
what is function?
A set of points in which every x value (domain) corresponds to exactly one y value (range)
Tim
what is lim (x,y)~(0,0) (x/y)
limited of x,y at 0,0 is nt defined
Alswell
But using L'Hopitals rule is x=1 is defined
Alswell
Could U explain better boss?
emmanuel
value of (x/y) as (x,y) tends to (0,0) also whats the value of (x+y)/(x^2+y^2) as (x,y) tends to (0,0)
NIKI
can we apply l hospitals rule for function of two variables
NIKI
why n does not equal -1
Andrew
I agree with Andrew
Bg
f (x) = a is a function. It's a constant function.
proof the formula integration of udv=uv-integration of vdu.?
Find derivative (2x^3+6xy-4y^2)^2
no x=2 is not a function, as there is nothing that's changing.
are you sure sir? please make it sure and reply please. thanks a lot sir I'm grateful.
The
i mean can we replace the roles of x and y and call x=2 as function
The
if x =y and x = 800 what is y
y=800
800
Bg
how do u factor the numerator?
Nonsense, you factor numbers
Antonio
You can factorize the numerator of an expression. What's the problem there? here's an example. f(x)=((x^2)-(y^2))/2 Then numerator is x squared minus y squared. It's factorized as (x+y)(x-y). so the overall function becomes : ((x+y)(x-y))/2
The
The problem is the question, is not a problem where it is, but what it is
Antonio
I think you should first know the basics man: PS
Vishal
Yes, what factorization is
Antonio
Antonio bro is x=2 a function?
The
Yes, and no.... Its a function if for every x, y=2.... If not is a single value constant
Antonio
you could define it as a constant function if you wanted where a function of "y" defines x f(y) = 2 no real use to doing that though
zach
Why y, if domain its usually defined as x, bro, so you creates confusion
Antonio
Its f(x) =y=2 for every x
Antonio
Yes but he said could you put x = 2 as a function you put y = 2 as a function
zach
F(y) in this case is not a function since for every value of y you have not a single point but many ones, so there is not f(y)
Antonio
x = 2 defined as a function of f(y) = 2 says for every y x will equal 2 this silly creates a vertical line and is equivalent to saying x = 2 just in a function notation as the user above asked. you put f(x) = 2 this means for every x y is 2 this creates a horizontal line and is not equivalent
zach
The said x=2 and that 2 is y
Antonio
that 2 is not y, y is a variable 2 is a constant
zach
So 2 is defined as f(x) =2
Antonio
No y its constant =2
Antonio
what variable does that function define
zach
the function f(x) =2 takes every input of x within it's domain and gives 2 if for instance f:x -> y then for every x, y =2 giving a horizontal line this is NOT equivalent to the expression x = 2
zach
Yes true, y=2 its a constant, so a line parallel to y axix as function of y
Antonio
Sorry x=2
Antonio
And you are right, but os not a function of x, its a function of y
Antonio
As function of x is meaningless, is not a finction
Antonio
yeah you mean what I said in my first post, smh
zach
I mean (0xY) +x = 2 so y can be as you want, the result its 2 every time
Antonio
OK you can call this "function" on a set {2}, but its a single value function, a constant
Antonio
well as long as you got there eventually
zach
2x^3+6xy-4y^2)^2 solve this
femi
moe
volume between cone z=√(x^2+y^2) and plane z=2
Fatima
It's an integral easy
Antonio
V=1/3 h π (R^2+r2+ r*R(
Antonio