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Introduction to Logarithms

What is a Logarithm?

Exponential equations are all about growth. For instance, $2^3 = 8$. If you start with 2 and multiply it by itself 3 times, you get 8.

But what if you know the starting number and the final number, but not the exponent? What if you have the equation $2^x = 8$ and need to find $x$? You can probably figure out in your head that $x$ is 3. But what about $2^x = 10$? That's not as easy.

Logarithms are the tool we use to solve for that missing exponent. A logarithm answers the question: "What exponent do I need to raise a specific base to, in order to get a certain number?"

Simply put, a logarithm is an exponent.

Notation and Meaning

The way we write a logarithm looks like this: logb(a)\log_b(a). Let's break down the parts.

PartNameQuestion it Asks
bbBaseWhat number are we multiplying?
aaArgumentWhat is the final number we want to get?
log\logThe operation that finds the missing exponent.

The entire expression, logb(a)\log_b(a), equals the exponent we are looking for. So, the relationship between exponents and logarithms is a two-way street. They are two different ways of saying the exact same thing.

logb(a)=c    bc=a\log_b(a) = c \iff b^c = a

Let's see this with our first example. To solve $2^x = 8$, we would write it as a logarithm:

$\log_2(8) = x$

This asks, "What power do we need to raise 2 to, in order to get 8?" As we know, the answer is 3. So, $\log_2(8) = 3$.

Here are a few more quick examples:

  • log5(25)=2\log_5(25) = 2, because 52=255^2 = 25.
  • log10(1000)=3\log_{10}(1000) = 3, because 103=100010^3 = 1000.
  • log3(9)=2\log_3(9) = 2, because 32=93^2 = 9.

The Inverse Relationship

Logarithms and exponents are inverse operations. This means they undo each other, just like subtraction undoes addition, and division undoes multiplication.

If you take a number, apply an exponential function to it, and then apply a logarithmic function with the same base, you get back to your original number.

This inverse relationship is also visible in their graphs. The graph of a logarithmic function is a reflection of its corresponding exponential function across the line y=xy=x.

Lesson image

Understanding this core concept, that a logarithm is just a way to find a missing exponent, is the first step. It's a fundamental tool for solving problems that involve rapid growth or decay.