Exponential function

The exponential function is the function $f(x) = e^x$, exponentiation by e. It is a very important function in analysis, both real and complex.

General Info and Definitions

Exponential functions are functions that grows or decays at a constant percent rate.

Exponential functions that result in an increase of y is called an exponential growth.
Exponential functions that result in an decrease of y is called an exponential decay.

An exponential growth graph looks like:

2 power x growth.jpg

An exponential decay graph looks like:

05 power x decay.jpg

Exponential functions are in one of three forms.

$f\left( x \right) = ab^x$, where b is the % change written in decimals
$f\left( x \right) = ae^k$, where e is the irrational constant 2.71828182846....
$f\left( x \right) = a\left( {{1 \over 2}} \right)^{{x \over h}}$ or $f\left( x \right) = a\left( 2 \right)^{{x \over d}}$, where h is the half-life (for decay), or d is the doubling time (for growth).

Whether an exponential function shows growth or decay depends upon the value of its b value.

If $b > 1$, then the function will show growth.
If $0 < b < 1$, then the function will show decay.

Solving Exponential Equations

There are two ways to solve an exponential equation. Graphically with a computer/calculator or algebraicly using logarithms.

Example: Solve $56 = 12\left( {1.24976} \right)^x$

  • Graphically:
Graph both equations and find the intersection.
Expfunc graphsolve eqn.jpg
  • Algebraically:

There, I will use natural logarithms. The same operation can also be done with common logarithms.

$56 = 12\left( {1.24976} \right)^x$
${{56} \over {12}} = \left( {1.24976} \right)^x$
$\ln \left( {{{56} \over {12}}} \right) = x\ln \left( {1.24976} \right)$
$x = {{\ln \left( {{{56} \over {12}}} \right)} \over {\ln \left( {1.24976} \right)}}$
$x \approx 6.9093$

Stay Connected

Subscribe to get news and
updates from AoPS, or Contact Us.
© 2015
AoPS Incorporated
Invalid username
Login to AoPS