Goal
Evaluate vector valued functions
Definition
A vector-valued function is a function of the form:
The functions f, g, h are real-valued functions of the parameter t. Vector-valued functions are also written in the form:
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Goal
Evaluate vector valued functions
Definition
A vector-valued function is a function of the form:
The functions f, g, h are real-valued functions of the parameter t. Vector-valued functions are also written in the form:
Let's consider the plane curve defined by the following parametric equations:
x = x(t) y = y(t) t₁≤t ≤t₂ and let's assume that x(t) and y(t) are differentiable functions of t, then the arc length of the parametric curve is given by:
This video gives an idea of where this formula originates:
Goal: Finding the area under the curve of a parametric equation
Area under parametric curve
Let's consider the the area of the curve bounded by the curve y = f(x), the x-axis and the verticals x = a and x = b
We know that the area is given by:
The formula that allows to find the area under a parametric curve is then given by:
Example
Find the area under curve of the cycloid define by the equations:
x(t) = t-sint y(t) = 1-cost 0 ≤ t ≤ 2π
Solution
Using the formula we have:
Practice
Find the area under the curve of the hypocycloid defined by the following equations:
x(t) = 3cost + cos3t y(t) = 3sint - sin3t 0 ≤ t ≤ π
The second order derivative of a function y = f(x) is the derivative of the first derivative of the function.
The first derivative of the function f is dy/dx.
The derivative of the first derivative is :d/dx[dy/dx] = d²y/dx²
The relation equality being commutative, we can write: d²y/dx² = d/dx[dy/dx].
Let's apply the formula of the first derivative. According to this formula, the derivative of y = f(x) is the derivative of the function y with respect to t divided by the derivative of x with respect to t.
The function here is dy/dx. Let's calculate its derivative:
d²y/dx² = d/dt[dy/dx]./dx/dt.
The second order derivative is the derivative of the derivative of the first derivative with respect to t divided by the derivative of x with respect to t.
Examples
Calculate the second derivative d²y/dx² for the plane curve defined by the parametric equations:
Solution
We have :
d²y/dx² = d/dt[dy/dx]./dx/dt.
Let's calculate dy/dx. According to the formula of the derivative, we know that the derivative is equal to the derivative of y with respect to t divided by the derivative of x with respect to t.
dy/dx = y'(t)/x'(t) = 2/2t = 1/t
Let's calculate dx/dt also:
dx/dt = 2t
The expression of d²y/dx² becomes:
d²y/dx² = d/dt (1/t)/2t = -1/t²/2t = (-1/t²)(1/2t) = -1/2t³
Practice
Calculate the second derivative d²y/dx² for the plane curve defined by the parametric equations:
Objective: to find the derivative of parametric curves
Derivatives of parametric equations
Let's consider the plane curve defined by the parametric equations:
x(t) = 2t + 3 y(t) = 3t - 4 -2≤ x≤3
Our objective is to determine the derivative of the parametric equations. Let's note that the derivative of a function is a function that represents the slopes of all the tangent lines at different points of the curve. If we know the slope of a tangent line at a point we can know the slopes of all the tangent lines at different points of the curve of the function. The slope of a tangent line at a point is the derivative of the function at a point. The function that allows to find the derivative at a point allows to determine the derivatives of all points of the function. Determining the slope of a tangent line to the parametric curve will allow us to determine the derivative of the parametric equations.
The curve of the parametric equations is a line starting at (-1, -10) and ending at (9, 5).
Let's substitute t in y(t):
But dy/dx = dy/dt/dx/dt = y'(t)/x'(t)
x'(t) = 2 y'(t) = 3
dy/dx = 3/2
In both cases dy/dt = 3/2. The expression dy/dx = y'(t)/x'(t) is the derivative of the function.
Theorem. Derivative of Parametric equations
Consider the plane curve defined by x = x(t) and y = y(t). Suppose that x'(t) and y'(t) exist, and then assume x'(t) ≠ 0. Then the derivative dy/dx is given by:
Examples
Calculate the derivative dy/dx for each of the parametrically defined curves and locate any critical points on their respective graph.
a) Let's calculate dy/dx: dy/dx = y'(t)/x'(t)
From the parametric equations in example a) we have: y'(t) = 2 and x'(t) = 2. Therefore:
dy/dx = 2/2t = 1/t
A critical point is a point where the derivative is equal to zero or undefined. The derivative is undefined for t = 0. Let's determine the critical point. Substituting t in the parametric equations, we find x = -3 and y = -1. (-3, -1) represents the critical point. The graph of the curve is a parabola opening on the right and the point (-3, -1) is a vertex of this parabola.
Since the denominator is different of zero, the only critical points are those for which the numerator is equal to zero. That happens when t = -1 or t = 1 for which the critical points are: (-1, 6) and (3, 2) respectively. Here is the graph of the parametric curve.
c) We have : dy/dx = -cost/sint
The derivative is 0 for cost = 0. That happens when t = π/2 or t = 3𝛑/2 to which correspond respectively the critical points (0,5) and (0, -5).
When sint = 0, the corresponding values of t are : t = 0, t = π, t = 2π. The critical points are respectively:
(5. 0), (-5, 0) and (5. 0).
Here is the graph of the parametric equations defined in part example c)
Calculate the derivative dy/dx for the plane curve defined by the following equations and locate any critical points on its graph:
Objective:
Transform the function of a curve into parametric equations.
Parameterization of a curve
In the previous lesson we learn how to eliminate the parameter in the parametric equations of a curve. In this post we do the reverse meaning transforming a function of a curve into parametric equations.
Example
Find two different pairs of parametric equations to represent the graph of y = 2x² - 3
Solution
One of the easiest way to do this is to write x(t) = t and substitute x in the function. The result is y = 2 t²-3.
The first set of pair of parametric equations is x(t) = t y = 2t²-3
Since there is no restriction on the domain of the function, we can have a variety of expressions of x in function of t and then substitute x in the function.
For the second pair of equations let's choose x(t) = 3t-2
Let's substitute x in the function:
Therefore a second parameterization of the function is represented by:
Find two different sets of parametric equations to represent the graph of y = x² + 2x
Objective: Convert the parametric equations of a curve into the form y = f(x)
Conversion of the parametric equations of a curve into an explicit form
Usually we are more accustomed in graphing the curve of a function represented explicitly meaning representing a relation between y and x. It is possible to convert the parametric equations of a curve into its explicit form. The process is very simple. It consist in finding the expression of t in one of the parametric equations and then substituting it in the other.
Examples:
a) let's solve example a)
Let's find t in the first equation. We can also find it in the second equation.
Let's substitute t in the second equation:
This is the equation of a parabola opening upward. The limits of the parameter lead to a domain restriction of the function.
The interval for t is mentioned in the problem: -6≤t≤-2
Let's substitute t = -6 in the expression of x, we find x = 0. Let's substitute t = -2 in the expression of x, we find x = 4. The domain of the function is D = [0,4].
Instead of solving for t find sint and cost in both equations. Substitute sint and cost in the identity sin²t + cos²t = 1. The equation found is the equation of an ellipse centered in the origin.
Practice
Eliminate the parameter for the plane curve defined by the following parametric equations and describe the resulting graph:
x(t) = 2 + 3/t y(t) = t-1 2≤t≤6
Objectives:
1. Define parametric equations
2. Plot their curve
Considerations
Let's consider the orbit of the earth around the sun.
Let's superimpose the ellipse on a system of coordinates (x,y):
Notice in this definition that x and y are used in two ways. The first is as functions of the independent variable t. As t varies over the interval I, the functions x(t) and y(t) generate a set of pairs (x,y). This set of ordered pairs generates the graph of the parametric equations. In this second usage, to designate the ordered pairs, x and y are variables. It is important to distinguish the variables x and y from the functions x(t) and y(t).
Objective: Solve differential equations using power series.
Let's consider the differential equation y'(x) = y. Recall that this is a first order separable equation and its solution is y = Ce^x. For most differential equations there are not any analytical tools to solve them. Power series are an extremely useful tool to solve them. The technique we use is to look for a solution of the form