If you"re taking a high school or college math class, you"ll likely cover natural logs. But what are natural logs? What is ln? Why does the letter e keep showing up?
Natural logs may seem difficult, but once you understand a few key natural log rules, you"ll be able khổng lồ easily solve even very complicated-looking problems. In this guide, we explain the four most important natural logarithm rules, discuss other natural log properties you should know, go over several examples of varying difficulty, và explain how natural logs differ from other logarithms.
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What Is ln?
The natural log, or ln, is the inverse of e. The letter ‘e" represents a mathematical constant also known as the natural exponent. Like π, e is a mathematical constant and has a mix value. The value of e is equal to lớn approximately 2.71828.
e appears in many instances in mathematics, including scenarios about compound interest, growth equations, & decay equations. Ln(x) is the time needed lớn grow to lớn x, while ex is the amount of growth that has occurred after time x.
Because e is used so commonly in math & economics, và people in these fields often need to take the logarithm with a base of e of a number khổng lồ solve an equation or find a value, the natural log was created as a shortcut way to lớn write and calculate log base e. The natural log simply lets people reading the problem know that you"re taking the logarithm, with a base of e, of a number. So ln(x) = loge(x). As an example, ln(5) = loge(5) = 1.609.
The 4 Key Natural Log Rules
There are four main rules you need to know when working with natural logs, & you"ll see each of them again & again in your math problems. Know these well because they can be confusing the first time you see them, & you want to lớn make sure you have basic rules like these down solid before moving on khổng lồ more difficult logarithm topics.
Product Rule
ln(x)(y) = ln(x) + ln(y)The natural log of the multiplication of x and y is the sum of the ln of x & ln of y.Example: ln(8)(6) = ln(8) + ln(6)Quotient Rule
ln(x/y) = ln(x) - ln(y)The natural log of the division of x & y is the difference of the ln of x and ln of y.Example: ln(7/4) = ln(7) - ln(4)Reciprocal Rule
ln(1/x) = −ln(x)The natural log of the reciprocal of x is the opposite of the ln of x.Example: ln(⅓)= -ln(3)Power Rule
ln(xy) = y * ln(x)The natural log of x raised lớn the nguồn of y is y times the ln of x.Example: ln(52) = 2 * ln(5)Key Natural Log Properties
In addition to lớn the four natural logarithm rules discussed above, there are also several ln properties you need khổng lồ know if you"re studying natural logs. Have these memorized so you can quickly move onto the next step of the problem without wasting time trying to remember common ln properties.
Scenario | ln Property |
ln of a Negative Number | The ln of a negative number is undefined |
ln of 0 | ln(0) is undefined |
ln of 1 | ln(1)=0 |
ln of Infinity | ln(∞)= ∞ |
ln of e | ln(e)=1 |
ln of e raised to lớn the x power | ln(ex) = x |
e raised to lớn the ln power | eln(x)=x |
As you can see from the final three rows, ln(e)=1, và this is true even if one is raised to the nguồn of the other. This is because the ln and e are inverse functions of each other.
Natural Log Sample Problems
Now it"s time lớn put your skills to lớn the test & ensure you understand the ln rules by applying them khổng lồ example problems. Below are three sample problems. Try to lớn work them out on your own before reading through the explanation.
Problem 1
Evaluate ln(72/5)
First, we use the quotient rule lớn get: ln(72) - ln(5).
Next, we use the nguồn rule lớn get: 2ln(7) -ln(5).
If you don"t have a calculator, you can leave the equation lượt thích this, or you can calculate the natural log values: 2(1.946) - 1.609 = 3.891 - 1.609 = 2.282.
Problem 2
Evaluate ln(e) /7
For this problem, we need to remember than ln(e)=1
This means the problem simplifies to 1/7, which is our answer
Problem 3
Solve ln (5x-6)=2
When you have multiple variables within the ln parentheses, you want to make e the base & everything else the exponent of e. Then you"ll get ln & e next khổng lồ each other and, as we know from the natural log rules, eln(x)=x.
So, the equation becomes eln(5x-6)=e2
Since eln(x)=x, eln(5x-6)= 5x-6
Therefore 5x-6= e2
Since e is a constant, you can then figure out the value of e2, either by using the e key on your calculator or using e"s estimated value of 2.718.
5x-6 =7.389
Now we"d add 6 lớn both sides
5x= 13.389
Finally, we"d divide both sides by 5.
x= 2.678
How Are Natural Logs Different From Other Logarithms?
As a reminder, a logarithm is the opposite of a power. If you take the log of a number, you"re undoing the exponent. The key difference between natural logs and other logarithms is the base being used. Logarithms typically use a base of 10 (although it can be a different value, which will be specified), while natural logs will always use a base of e.
This means ln(x)=loge(x)
If you need lớn convert between logarithms và natural logs, use the following two equations:
log10(x) = ln(x) / ln(10)ln(x) = log10(x) / log10(e)Other than the difference in the base (which is a big difference) the logarithm rules & the natural logarithm rules are the same:
Logarithm Rules | ln Rules |
log(xy)=log(x)+log(y) | ln(x)(y)= ln(x)+ln(y) |
log(x/y)=log(x)−log(y) | ln(x/y)=ln(x)−ln(y) |
log(xa)= alog(x) | ln(xa)= aln(x) |
log(10x)= x | ln(ex)= x |
10log(x)= x | eln(x)= x |
Summary: Natural Log Rules
The natural log, or ln, is the inverse of e. The rules of natural logs may seem counterintuitive at first, but once you learn them they"re quite simple to lớn remember and apply lớn practice problems.
The four main ln rules are:
ln(x)( y) = ln(x) + ln(y)ln(x/y) = ln(x) - ln(y)ln(1/x)=−ln(x)n(xy) = y*ln(x)The key difference between natural logs and other logarithms is the base being used.
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