Calculus 2 : Graphing Parametrics

Study concepts, example questions & explanations for Calculus 2

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Example Questions

Example Question #1 : Graphing Parametrics

Suppose we have a curve parameterized by the equations:

\displaystyle x = e^{-t}

\displaystyle y = t^3 - 3t^2 + 1

What is the tangent line to the curve at \displaystyle t = 2?

Possible Answers:

\displaystyle y = -3

\displaystyle y = x + e

\displaystyle y = e^2 x - 4

\displaystyle y = -e x + 1

\displaystyle y = -3x -3 + 3e^{-2}

Correct answer:

\displaystyle y = -3

Explanation:

At \displaystyle t=2, the graph passes through \displaystyle (e^{-2}, 2^3 - 3\cdot 2^2 + 1) = (e^{-2}, -3)

Now to find the slope, we will need both derivatives with respect to t, which are:

\displaystyle \frac{dx}{dt} = -e^{-t}

\displaystyle \frac{dy}{dt} = 3t^2 - 6t

So to obtain the slope, we just use:

 \displaystyle \frac{\mathrm{dy} }{\mathrm{d} x} = \frac{dy/dt}{dx/dt},

and evaluate at \displaystyle t=2.

As it turns out, \displaystyle \frac{\mathrm{dy} }{\mathrm{d} t} at \displaystyle t=2, and \displaystyle \frac{\mathrm{dx} }{\mathrm{d} t} \neq 0, so the slope will be 0 for this curve at the point \displaystyle t=2.

That means that \displaystyle y = 0x +b = b, and so solving at ordered pair \displaystyle (e^{-2}, -3), the solution must be:

\displaystyle y = -3

Example Question #612 : Calculus Ii

Describe the graph of the following set of parametric equations:

\displaystyle x = 4 sin(t) - 2

\displaystyle y = 9cos(t) + 1

Possible Answers:

A circle, centered at \displaystyle (2,-1) with a radius of \displaystyle \frac{9}{8}.

A sinusoidal graph with amplitude \displaystyle \frac{2}{3}, shifted up one unit and left two units.

An ellipse, centered at \displaystyle (-2, 1) with horizontal axis \displaystyle 4 and vertical axis \displaystyle 9.

A circle, centered at \displaystyle (2,-1) with a radius of \displaystyle \frac{9}{4}.

An ellipse, centered at \displaystyle (-2, 1) with horizontal axis \displaystyle 2 and vertical axis \displaystyle 3.

Correct answer:

An ellipse, centered at \displaystyle (-2, 1) with horizontal axis \displaystyle 4 and vertical axis \displaystyle 9.

Explanation:

\displaystyle x = 4 sin(t) - 2

\displaystyle y = 9cos(t) + 1

Perform these operations:

\displaystyle x + 2 = 4sin (t) \Rightarrow \frac{x+2}{4} = sin(t)

\displaystyle y - 1 = 9 cos(t) \Rightarrow \frac{y-1}{9} = cos(t)

Now, we can use a Pythagorean trigonometric identity to transform the equation into a rectangular equation:

 

\displaystyle \frac{(x+2)^2}{4^2} + \frac{(y-1)^2}{9^2} = sin^2t + cos^2 t = 1

And this is the equation of an ellipse, centered at \displaystyle (-2, 1) with horizontal axis \displaystyle 4 and vertical axis \displaystyle 9.

 

 

Example Question #3 : Graphing Parametrics

Given \displaystyle x=-t+5 and \displaystyle y=9t+4, what is \displaystyle y in terms of \displaystyle x (rectangular form)?

Possible Answers:

\displaystyle y=49-9x

\displaystyle y=9x-49

None of the above

\displaystyle y=49+9x

\displaystyle y=9x+49

Correct answer:

\displaystyle y=49-9x

Explanation:

Given \displaystyle x=-t+5 and \displaystyle y=9t+4 let's solve both equations for \displaystyle t:

\displaystyle x=-t+5\rightarrow t=5-x

\displaystyle y=9t+4\rightarrow t=\frac{y-4}{9}

Since both equations equal \displaystyle t, let's set them equal to each other and solve for \displaystyle y:

\displaystyle \frac{y-4}{9}=5-x

\displaystyle y-4=9(5-x)

\displaystyle y=9(5-x)+4

\displaystyle y=45-9x+4

\displaystyle y=49-9x

 

Example Question #4 : Graphing Parametrics

Given \displaystyle x=2t+7 and \displaystyle y=7t-2, what is the length of the arc from \displaystyle 0\leq t\leq2?

Possible Answers:

\displaystyle 3\sqrt{53}

\displaystyle \sqrt{53}

\displaystyle 6\sqrt{53}

\displaystyle 2\sqrt{53}

\displaystyle 5\sqrt{53}

Correct answer:

\displaystyle 2\sqrt{53}

Explanation:

In order to find the arc length, we must use the arc length formula for parametric curves:

\displaystyle L=\int_{a}^{b}\sqrt{\left(\frac{dx}{dt}\right)^{2}+\left(\frac{dy}{dt}\right)^{2}}dt.

Given \displaystyle x=2t+7 and \displaystyle y=7t-2, we can use using the Power Rule

for all ,

to derive 

\displaystyle \frac{dx}{dt}=(1)2t^{1-1}+(0)7=2 and \displaystyle \frac{dy}{dt}=(1)7t^{1-1}-(0)2=7.

Plugging these values and our boundary values for \displaystyle t into the arc length equation, we get:

\displaystyle L=\int_{0}^{2}\sqrt{(2)^{2}+(7)^{2}}dt

\displaystyle L=\int_{0}^{2}(\sqrt{4+49})dt

\displaystyle L=\int_{0}^{2}(\sqrt{53})dt

Now, using the Power Rule for Integrals \displaystyle \int x^{n}dx=\frac{x^{n+1}}{n+1} for all \displaystyle n\neq0, we can determine that:

\displaystyle L=\int_{0}^{2}(\sqrt{53})dt

\displaystyle L=[t\sqrt{53}]_{0}^{2}\textrm{}

\displaystyle L=2\sqrt{53}-0\sqrt{53}

\displaystyle L=2\sqrt{53}

 

Example Question #2 : Graphing Parametrics

Find \displaystyle \frac{dy}{dx} using the following parametric equations

\displaystyle x(t)=t^2+t^3, y(t)=e^{2t}.

Possible Answers:

\displaystyle \frac{dy}{dx}=\frac{2e^{2t}}{3t^2+2t}

\displaystyle \frac{dy}{dx}=3t^2+2t

\displaystyle \frac{dy}{dx}=2e^{2t}

\displaystyle \frac{dy}{dx}=\frac{3t^2+2t}{2e^{2t}}

Correct answer:

\displaystyle \frac{dy}{dx}=\frac{2e^{2t}}{3t^2+2t}

Explanation:

It is known that we can derive \displaystyle \frac{dy}{dx} with the formula

\displaystyle \frac{dy}{dx}=\frac{dy/dt}{dx/dt}

So we just find \displaystyle \frac{dy}{dt}, \frac{dx}{dt}:

To find these derivatives we will need to use the power rule, chain rule, and rule of exponentials.

Power Rule: \displaystyle x^n\ dx =nx^{n-1}

Chain Rule: \displaystyle f(g(x))\ dx=f'(g(x))\cdot g'(x)

Rules of Exponentials: \displaystyle e^u dx=e^u \cdot \frac{du}{dx}

Applying these rules we get the following.

\displaystyle \frac{dy}{dt}=2e^{2t}

\displaystyle \frac{dx}{dt}=3t^2+2t

so we have 

\displaystyle \frac{dy}{dx}=\frac{2e^{2t}}{3t^2+2t}.

Example Question #101 : Parametric

Given the parametric equations

\displaystyle x(t)=\cos t, y(t)=\ln t

find \displaystyle \frac{dy}{dx}.

Possible Answers:

\displaystyle \frac{dy}{dx}=-\frac{t}{\sin t}

\displaystyle \frac{dy}{dx}=-\frac{1}{t\sin t}

\displaystyle \frac{dy}{dx}=-\sin t

\displaystyle \frac{dy}{dx}=-t\sin t

Correct answer:

\displaystyle \frac{dy}{dx}=-\frac{1}{t\sin t}

Explanation:

It is known that we can derive \displaystyle \frac{dy}{dx} with the formula

\displaystyle \frac{dy}{dx}=\frac{dy/dt}{dx/dt}

So we just find \displaystyle \frac{dy}{dt}, \frac{dx}{dt}:

To find the derivatives we will need to use trigonometric rules and the rule for natural logs.

Trigonometric Rule for cosine: \displaystyle cos(t)dt=-sin(t)

Rule of Natural Log: \displaystyle ln(u)dt=\frac{1}{u}\cdot \frac{du}{dt}

Applying the above rules we get the following derivatives.

\displaystyle \frac{dy}{dt}=\frac{1}{t}

\displaystyle \frac{dx}{dt}=-\sin t

so we have 

\displaystyle \frac{dy}{dx}=-\frac{1}{t\sin t}.

Example Question #101 : Parametric, Polar, And Vector

Graph the following parametric equation:

\displaystyle x=\sec(t), y=\tan(t), t\in(-\infty,\infty)

Possible Answers:

Xsquaredplusysquared 1

Xsquaredminusysquared 1

Ysquaredminusxsquared 1

None of the other answers

Y 1overx 2

Correct answer:

Xsquaredminusysquared 1

Explanation:

Using the identity \displaystyle \sec^2(t)-\tan^2(t)=1, we can plug in the values \displaystyle x for \displaystyle \sec(t) and \displaystyle y for \displaystyle \tan(t) to obtain the equation \displaystyle x^2-y^2=1. This is the graph of a horizontal hyperbola with x-intercepts of \displaystyle (-1,0) and \displaystyle (1,0) with asympotes of \displaystyle y=\pm{x}. The picture is depicted below:

Xsquaredminusysquared 1

Example Question #611 : Calculus Ii

In which quadrant does the parametric equation terminate when \displaystyle t=\frac{\pi}{4} ?

\displaystyle x=cos\,t

\displaystyle y=sin\,t

Possible Answers:

\displaystyle II

\displaystyle IV

\displaystyle I

\displaystyle III

Correct answer:

\displaystyle I

Explanation:

When \displaystyle t=\frac{\pi}{4}

we have that

\displaystyle x=cos(\frac{\pi}{4})=\frac{\sqrt2}{2}

\displaystyle y=sin(\frac{\pi}{4})=\frac{\sqrt2}{2}

This gives the coordinate

\displaystyle (\frac{\sqrt2}{2},\frac{\sqrt2}{2})

which is located in

\displaystyle Quadrant\, I

Example Question #1 : Graphing Parametrics

In which quadrant does the parametric equation terminate when \displaystyle t=3 ?

\displaystyle x=8t

\displaystyle y=-4t

Possible Answers:

\displaystyle III

\displaystyle II

\displaystyle IV

\displaystyle I

Correct answer:

\displaystyle IV

Explanation:

When \displaystyle t=3

we have that

\displaystyle x=8(3)=24

\displaystyle y=-4(3)=-12

This gives the coordinate

\displaystyle (24,-12)

which is located in

\displaystyle Quadrant\, IV

 
 

Example Question #10 : Graphing Parametrics

In which quadrant is the parametric function located for the given \displaystyle t value?

\displaystyle (3t,\, t^3)

\displaystyle t=-2

Possible Answers:

\displaystyle III

\displaystyle II

\displaystyle IV

\displaystyle I

Correct answer:

\displaystyle III

Explanation:

We substitute the given \displaystyle t value into the parametric function.

\displaystyle (3(-2), \,(-2)^3)

\displaystyle =(-6,-8)

The resulting coordinate is in

\displaystyle Quadrant\,III

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