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IB DP Maths AA SL Study Notes

5.2.1 Equation of a Tangent Line

Point-Slope Form

The point-slope form is an efficient way to represent the equation of a line, especially when a specific point on the line and its slope are known. The general formula for the point-slope form is:

y - y1 = m(x - x1)

Where:

  • (x1, y1) is a known point on the line.
  • m is the slope of the line.

For the equation of a tangent line to a curve at a given point, the slope m is the derivative of the function at that point. To understand how the normal line relates to the tangent line, see 5.2.2 Normal Line.

Understanding the Point-Slope Form

To grasp the point-slope form, let's break down its components:

  • Point on the Line: Every line can be defined by any point on it. In the context of the tangent line, this point is where the line touches the curve.
  • Slope: The slope represents the steepness of the line. For a tangent line, the slope is the rate of change of the function at the point of tangency. In calculus, this rate of change is given by the derivative. Understanding the 5.1.1 Power Rule is crucial for finding derivatives efficiently.

Example:

Consider the function f(x) = x2. Let's determine the equation of the tangent line to this curve at the point (1,1).

1. First, differentiate f(x):

f'(x) = 2x

2. Evaluate the derivative at x = 1 to get the slope of the tangent:

m = f'(1) = 2

3. Using the point-slope form:

y - 1 = 2(x - 1)

This equation represents the tangent line to the curve y = x2 at the point (1,1).

For more on derivative applications, see the 5.1.3 Chain Rule.

Applications of the Tangent Line

The tangent line is not just a theoretical concept; it has practical applications in various fields:

1. Function Approximation

The tangent line can approximate the value of a function near a specific point. This is especially useful when dealing with intricate functions where exact values might be challenging to compute.

2. Curve Analysis

The slope of the tangent line provides insights into the behaviour of the curve. A positive slope indicates the function is increasing, while a negative slope suggests it's decreasing. A zero slope means the function is neither increasing nor decreasing at that point. Learn more about curve behaviour with 5.6.1 Position and Velocity.

3. Real-world Applications

  • Physics: In physics, the tangent line can represent instantaneous velocity when dealing with position-time graphs.
  • Engineering: Engineers use it to determine stresses and strains on materials.
  • Economics: Economists use tangent lines to find cost minimisation and profit maximisation points. For a related topic, check out 3.1.1 Introduction to Radians.

Example:

Imagine a car's position is described by the function s(t) = t3 - 6t2 + 9t, where s is the distance in metres and t is the time in seconds. Determine the car's velocity at t = 2 seconds.

1. Velocity is the derivative of the position function:

v(t) = s'(t) = 3t2 - 12t + 9

2. Evaluate v(t) at t = 2:

v(2) = 3(22) - 12(2) + 9 = 3

Thus, the car's velocity at t = 2 seconds is 3 m/s.

Practice Questions

1. Determine the equation of the tangent line to the curve y = x3 at the point (2,8).

2. A particle moves along a trajectory described by s(t) = 4t2 - t3. Ascertain its velocity at t = 3 seconds.

FAQ

A tangent line and a secant line are both straight lines associated with a curve, but they serve different purposes and have distinct definitions. A tangent line touches a curve at a single point and provides an approximation of the curve's behaviour around that point. On the other hand, a secant line intersects a curve at two distinct points. The slope of the secant line between two points on a curve represents the average rate of change of the function over that interval. As the two points get closer and closer, the secant line approaches the tangent line, and its slope approaches the instantaneous rate of change, given by the derivative.

No, there can only be one unique tangent line to a curve at a specific point. By definition, a tangent line "touches" the curve at only one point and does not cross it at that point. If there were more than one tangent line at a single point, it would contradict this definition. However, it's worth noting that some curves might have points where a tangent line doesn't exist, such as sharp corners or cusps. At such points, the curve's behaviour changes too abruptly for a single tangent line to provide a good approximation.

The slope of the tangent line to a curve at a particular point is precisely the value of the derivative of the function at that point. In calculus, the derivative of a function represents the rate of change of the function. When we talk about the rate of change at a specific point, we're essentially referring to the slope of the tangent line at that point. Therefore, by calculating the derivative of a function and evaluating it at a given point, we can determine the slope of the tangent line to the curve at that point.

The concept of a tangent line is closely related to linear approximation. Linear approximation is a method used to approximate the value of a function near a specific point using the tangent line to the curve at that point. Since the tangent line represents the local behaviour of the curve around the point of tangency, it can be used as a straight-line approximation of the function in that vicinity. Mathematically, the equation of the tangent line can be used to estimate the function's values close to the point of tangency. This is particularly useful when dealing with complex functions where exact computation might be challenging or time-consuming.

The tangent line is crucial in real-world applications because it provides an approximation of a function's behaviour near a specific point. For instance, in engineering and physics, the tangent line can represent the instantaneous rate of change, such as velocity or acceleration. In economics, it can be used to determine the rate of profit or cost changes. The tangent line's slope gives insights into whether a function is increasing, decreasing, or constant at a particular point, which can be pivotal in decision-making processes. Moreover, the tangent line simplifies complex curves into straight lines, making them easier to analyse and interpret in practical scenarios.

Practice Questions

A particle's trajectory is described by s(t) = t^2 - 4t + 3. Determine the equation of the tangent to its trajectory at t = 2 seconds.

To determine the equation of the tangent to the particle's trajectory, we first need to find the slope of the tangent at the given point. This is given by the derivative of the position function. Differentiating s(t) = t2 - 4t + 3 with respect to t, we obtain:

s'(t) = 2t - 4

Evaluating this at t = 2, we get:

s'(2) = 2(2) - 4 = 0

Thus, the slope of the tangent at t = 2 is 0. Using the point-slope form, and knowing that the point is (2, -1), the equation of the tangent line is:

y - (-1) = 0(t - 2) y = -1

Consider the function y = 2x^3 - 3x^2 + x. Determine the equation of the tangent line to this curve at the point where x = 1.

To find the equation of the tangent line, we first need to determine the slope of the tangent. This is given by the derivative of the function. Differentiating y = 2x3 - 3x2 + x with respect to x, we get:

y' = 6x2 - 6x + 1

Evaluating this at x = 1, we get:

y'(1) = 6(12) - 6(1) + 1 = 1

Thus, the slope of the tangent at x = 1 is 1. Using the point-slope form, and knowing that the point is (1, 2(13) - 3(12) + 1), the equation of the tangent line is:

y - 0 = 1(x - 1) y = x

Dr Rahil Sachak-Patwa avatar
Written by: Dr Rahil Sachak-Patwa
LinkedIn
Oxford University - PhD Mathematics

Rahil spent ten years working as private tutor, teaching students for GCSEs, A-Levels, and university admissions. During his PhD he published papers on modelling infectious disease epidemics and was a tutor to undergraduate and masters students for mathematics courses.

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