Linear approximation

In mathematics, a linear approximation is a way to guess the value of a hard-to-calculate function by using a simpler, straight-line function. This helps us make good guesses when dealing with tough math problems. By using a linear function, which is just a straight line, and an affine function, we can estimate how a more complicated function behaves near a certain point.

Linear approximations are useful in many areas, especially for solving equations that are hard to solve directly. They are important in the method of finite differences, which helps break down complex problems into simple steps. This makes it easier to understand and solve difficult math problems.

Definition

A linear approximation is a way to guess the value of a function using a straight line. Imagine you look at a curve very closely; it starts to look like a straight line. This straight line is called the tangent line, and it touches the curve at one point.

To make this guess, we use the value of the function at a specific point and the slope of the curve at that point. This gives us a good guess when we are close to that point. If the curve bends sharply, the guess might not be as good, but it still works well near the chosen point.

Linear approximations can also be used for functions with more than one input, where the slope is replaced by a special kind of matrix. This helps us understand how the function changes when we move a little bit in different directions.

Applications

Optics

Main article: Gaussian optics

Gaussian optics helps us understand how light works in tools like microscopes and telescopes. It uses a special way of thinking called the paraxial approximation. This means we only look at light rays that are close to the main axis of the tool. With this, we can use simple math to describe things like how far the light focuses (focal distance), how big the image is (magnification), and how bright it is (brightness). This works well for tools with flat or curved surfaces, like lenses made from parts of a sphere.

Period of oscillation

Main article: Pendulum

A pendulum's time to swing back and forth depends on how long it is, how strong gravity is, and a little on how far it swings from its resting spot. But if we watch a pendulum swing only a little bit, we can use a simple math trick to find its time. In this case, the time it takes to swing doesn't change much no matter how big the swing is. This makes pendulums good for telling time, because each swing takes the same amount of time, even if the swings get bigger or smaller.

Electrical resistivity

Main article: Electrical resistivity

Most materials let electricity flow differently depending on their temperature. When the temperature doesn't change too much, we can use a simple math trick to guess how well the material will let electricity flow. This trick works only when the temperature stays close to a certain fixed temperature, usually room temperature. If the temperature changes a lot, this simple trick isn't good enough, and we need more complicated math to understand what happens.

T ≈ 2 π L g θ 0 ≪ 1 {\displaystyle T\approx 2\pi {\sqrt {\frac {L}{g}}}\qquad \qquad \qquad \theta _{0}\ll 1}

Example

We can use a straight line to guess the answer to some hard math problems. This is called a linear approximation.

For example, let’s look at the function f(x) = √(x + 3) and find its linear approximation at the point where x = 1.

We find that the linear approximation is √(x + 3) ≈ 7/4 + x/4.

Using this, we can guess:

√3.98 ≈ 1.995
√4.05 ≈ 2.0125