# 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.

find the nth differential coefficient of cosx.cos2x.cos3x
determine the inverse(one-to-one function) of f(x)=x(cube)+4 and draw the graph if the function and its inverse
f(x) = x^3 + 4, to find inverse switch x and you and isolate y: x = y^3 + 4 x -4 = y^3 (x-4)^1/3 = y = f^-1(x)
Andrew
in the example exercise how does it go from -4 +- squareroot(8)/-4 to -4 +- 2squareroot(2)/-4 what is the process of pulling out the factor like that?
Andrew
√(8) =√(4x2) =√4 x √2 2 √2 hope this helps. from the surds theory a^c x b^c = (ab)^c
Barnabas
564356
Myong
can you determine whether f(x)=x(cube) +4 is a one to one function
Crystal
one to one means that every input has a single output, and not multiple outputs. whenever the highest power of a given polynomial is odd then that function is said to be odd. a big help to help you understand this concept would be to graph the function and see visually what's going on.
Andrew
one to one means that every input has a single output, and not multiple outputs. whenever the highest power of a given polynomial is odd then that function is said to be odd. a big help to help you understand this concept would be to graph the function and see visually what's going on.
Andrew
can you show the steps from going from 3/(x-2)= y to x= 3/y +2 I'm confused as to how y ends up as the divisor
step 1: take reciprocal of both sides (x-2)/3 = 1/y step 2: multiply both sides by 3 x-2 = 3/y step 3: add 2 to both sides x = 3/y + 2 ps nice farcry 3 background!
Andrew
first you cross multiply and get y(x-2)=3 then apply distribution and the left side of the equation such as yx-2y=3 then you add 2y in both sides of the equation and get yx=3+2y and last divide both sides of the equation by y and you get x=3/y+2
Ioana
Multiply both sides by (x-2) to get 3=y(x-2) Then you can divide both sides by y (it's just a multiplied term now) to get 3/y = (x-2). Since the parentheses aren't doing anything for the right side, you can drop them, and add the 2 to both sides to get 3/y + 2 = x
Melin
thank you ladies and gentlemen I appreciate the help!
Robert
keep practicing and asking questions, practice makes perfect! and be aware that are often different paths to the same answer, so the more you familiarize yourself with these multiple different approaches, the less confused you'll be.
Andrew
please how do I learn integration
they are simply "anti-derivatives". so you should first learn how to take derivatives of any given function before going into taking integrals of any given function.
Andrew
best way to learn is always to look into a few basic examples of different kinds of functions, and then if you have any further questions, be sure to state specifically which step in the solution you are not understanding.
Andrew
example 1) say f'(x) = x, f(x) = ? well there is a rule called the 'power rule' which states that if f'(x) = x^n, then f(x) = x^(n+1)/(n+1) so in this case, f(x) = x^2/2
Andrew
great noticeable direction
Isaac
limit x tend to infinite xcos(π/2x)*sin(π/4x)
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.