Calculus 2 : Derivatives of Parametrics

Study concepts, example questions & explanations for Calculus 2

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Example Questions

Example Question #1 : Derivatives Of Parametrics

Find the derivative of the following set of parametric equations:

\(\displaystyle x=2t^3+5t^2-11t+3\)

\(\displaystyle y=7t^2-4t-18\)

Possible Answers:

\(\displaystyle \frac{t^2-7t+8}{6t-2}\)

\(\displaystyle \frac{2(7t-2)}{6t^2+10t-11}\)

\(\displaystyle \frac{-14t+1}{8t^2+5t-2}\)

\(\displaystyle \frac{5t^2+6t+3}{2t-11}\)

\(\displaystyle \frac{4(3t+4)}{2t^2+4t-3}\)

Correct answer:

\(\displaystyle \frac{2(7t-2)}{6t^2+10t-11}\)

Explanation:

We start by taking the derivative of x and y with respect to t, as both of the equations are only in terms of this variable:

\(\displaystyle \frac{dx}{dt}=6t^2+10t-11\)

\(\displaystyle \frac{dy}{dt}=14t-4\)

The problem asks us to find the derivative of the parametric equations, dy/dx, and we can see from the work below that the dt term is cancelled when we divide dy/dt by dx/dt, leaving us with dy/dx:

\(\displaystyle \frac{\frac{dy}{dt}}{\frac{dx}{dt}}=\frac{dy}{dt}\frac{dt}{dx}=\frac{dy}{dx}\)

So now that we know dx/dt and dy/dt, all we must do to find the derivative of our parametric equations is divide dy/dt by dx/dt:

\(\displaystyle \frac{dy}{dx}=\frac{\frac{dy}{dt}}{\frac{dx}{dt}}=\frac{14t-4}{6t^2+10t-11}=\frac{2(7t-2)}{6t^2+10t-11}\)

Example Question #1 : Derivatives Of Parametrics

Solve:

\(\displaystyle \int sin^3xcos^4x dx\)

Possible Answers:

\(\displaystyle \frac{1}{5} sin^5x - \frac{1}{6} sin^6x + C\)

\(\displaystyle \frac{1}{4} sin^4x + \frac{1}{7} sin^8x + C\)

\(\displaystyle -\frac{1}{5} cos^5x + \frac{1}{7} cos^7x + C\)

\(\displaystyle \frac{1}{5} sin^5x - \frac{1}{7} cos^5x + C\)

\(\displaystyle \frac{1}{4} cos^4x - \frac{1}{6} cos^6x + C\)

Correct answer:

\(\displaystyle -\frac{1}{5} cos^5x + \frac{1}{7} cos^7x + C\)

Explanation:

The integration involves breaking up a power of a trigonometric ratio, and then using known trigonometric identities.

\(\displaystyle \int sin^3x cos^4x dx = \int [sin x \cdot sin^2x \cdot cos^4x]dx\)\(\displaystyle = \int \left[sin x \cdot \left(\frac{1}{2} - \frac{1}{2}cos 2x\right)\cdot cos^4x\right] dx\)

\(\displaystyle = \frac{1}{2}\int [sin x cos^4x - sin x cos 2x cos^4x]dx\)

\(\displaystyle = \frac{1}{2} \int [sin x cos^4x - sin x (2cos^2x - 1)cos^4x ]dx\)

\(\displaystyle = \frac{1}{2} \int [2sin x cos^4x - 2sin x cos^6x]dx\)

\(\displaystyle = \frac{1}{2} \cdot -\frac{1}{5} \cdot 2cos^5x - \frac{1}{2} \cdot 2 \cdot- \frac{1}{7}cos^7x + C\)

\(\displaystyle = -\frac{1}{5} cos^5x + \frac{1}{7} cos^7x + C\)

 

The alternative is to find which answer choice has a derivative equal to the answer choice, and for this we get:

\(\displaystyle {}\frac{\mathrm{d} }{\mathrm{d} x}\left(-\frac{1}{5} cos^5x + \frac{1}{7} cos^7x + C\right) = cos^4x\cdot sin(x) - cos^6x\cdot sin(x)\)

\(\displaystyle {}= cos^4xsin(x)[1 - cos^2x] = cos^4x\cdot sin(x)\cdot sin^2x = sin^3xcos^4x\)

 

 

Example Question #1 : Derivatives Of Parametrics

Solve for \(\displaystyle \frac{dy}{dx}\) if \(\displaystyle x=2t^3\) and \(\displaystyle y=3t^4\).

Possible Answers:

\(\displaystyle 2t\)

\(\displaystyle \frac{3}{2}\)

\(\displaystyle 6t\)

\(\displaystyle 2\)

\(\displaystyle \frac{3}{2}t\)

Correct answer:

\(\displaystyle 2t\)

Explanation:

Write the the formula to solve for the derivative of parametric functions.

\(\displaystyle \large \frac{dy}{dx}=\frac{\frac{dy}{dt}}{\frac{dx}{dt}}\)

Find \(\displaystyle \frac{dy}{dt}\)and \(\displaystyle \frac{dx}{dt}\) using the power rule \(\displaystyle f(x)=x^n \rightarrow f'(x)=nx^{n-1}\).

\(\displaystyle \frac{dy}{dt}=12t^3\)

\(\displaystyle \frac{dx}{dt}=6t^2\)

Substitute back to the formula to obtain the derivative.

\(\displaystyle {} \frac{dy}{dx}=\frac{\frac{dy}{dt}}{\frac{dx}{dt}}=\frac{12t^3}{6t^2}=2t\)

Example Question #2 : Derivatives Of Parametrics

Find the derivative of the following parametric function:

\(\displaystyle x=t^2-1, y=\cos(t)+\sin(t)\)

 

Possible Answers:

\(\displaystyle \cos(t)-\sin(t)\)

\(\displaystyle \frac{\sin(t)-\cos(t)}{2t}\)

\(\displaystyle \frac{2t}{\cos(t)-\sin(t)}\)

\(\displaystyle \frac{\cos(t)-\sin(t)}{2t}\)

Correct answer:

\(\displaystyle \frac{\cos(t)-\sin(t)}{2t}\)

Explanation:

The derivative of a parametric function is given by:

\(\displaystyle \frac{dy}{dx}=\frac{\frac{dy}{dt}}{\frac{dx}{dt}}\)

where

\(\displaystyle \frac{dy}{dt}=\cos(t)-\sin(t)\), \(\displaystyle \frac{dx}{dt}=2t\)

The derivatives were found using the following rules:

\(\displaystyle \frac{d}{dx}\cos(x)=-\sin(x)\)

\(\displaystyle \frac{d}{dx}\sin(x)=\cos(x)\)

\(\displaystyle \frac{d}{dx}(x^n)nx^{n-1}\)

Simply divide the derivatives as shown above.

Example Question #1 : Derivatives Of Parametrics

Solve for \(\displaystyle \frac{dy}{dx}\) if \(\displaystyle y=3t^{2}-2\) and \(\displaystyle x=4t^{2}+6\).

Possible Answers:

\(\displaystyle \frac{4}{3}t\)

None of the above

\(\displaystyle \frac{3}{4}\)

\(\displaystyle \frac{3}{4}t\)

\(\displaystyle \frac{4}{3}\)

Correct answer:

\(\displaystyle \frac{3}{4}\)

Explanation:

Given equations for \(\displaystyle y\) and \(\displaystyle x\) in terms of \(\displaystyle t\), we can find the derivative of parametric equations as follows:

\(\displaystyle \frac{dy}{dx}=\frac{\frac{dy}{dt}}{\frac{dx}{dt}}\), as the \(\displaystyle dt\) terms will cancel out.

Using the Power Rule

\(\displaystyle \frac{d}{dx}x^{n}=nx^{n-1}\) for all \(\displaystyle n\neq0\) and given \(\displaystyle y=3t^{2}-2\) and \(\displaystyle x=4t^{2}+6\),

\(\displaystyle \frac{dy}{dt}=(2)3t^{2-1}-(0)2=6t\)  and \(\displaystyle \frac{dx}{dt}=(2)4t^{2-1}+(0)6=8t\).

Therefore, 

\(\displaystyle \frac{dy}{dx}=\frac{\frac{dy}{dt}}{\frac{dx}{dt}}=\frac{6t}{8t}=\frac{3}{4}\).

 

 

Example Question #3 : Derivatives Of Parametrics

Find the derivative of the following parametric equation:

\(\displaystyle x=2t+\cos(t), y=t^2\)

Possible Answers:

\(\displaystyle \frac{2+\sin(t)}{2t}\)

\(\displaystyle \frac{2t}{2-\sin(t)}\)

\(\displaystyle \frac{2-\sin(t)}{2t}\)

\(\displaystyle \frac{2t}{2+\sin(t)}\)

Correct answer:

\(\displaystyle \frac{2t}{2-\sin(t)}\)

Explanation:

The derivative of a parametric equation is given by the following equation:

\(\displaystyle \frac{dy}{dx}=\frac{\frac{dy}{dt}}{\frac{dx}{dt}}\)

Solving for the derivative of the equation for y, you get

\(\displaystyle \frac{dy}{dt}=2t\)

and for the equation for x, you get

\(\displaystyle \frac{dx}{dt}=2-\sin(t)\)

The following rules were used for the derivatives:

\(\displaystyle \frac{d}{dx}(x^n)=nx^{n-1}\)\(\displaystyle \frac{d}{dx}\cos(x)=-\sin(x)\)

Example Question #6 : Derivatives Of Parametrics

Find \(\displaystyle \frac{dx}{dy}\) if \(\displaystyle x=cos(t)\) and \(\displaystyle y=sin(t)\).

Possible Answers:

\(\displaystyle cot(t)\)

\(\displaystyle -cot(t)\)

\(\displaystyle -tan(t)\)

\(\displaystyle sec(t)\)

\(\displaystyle tan(x)\)

Correct answer:

\(\displaystyle -tan(t)\)

Explanation:

Write the formula to find the derivative for parametric equations.

\(\displaystyle \frac{dx}{dy}=\frac{\frac{dx}{dt}}{\frac{dy}{dt}}\)

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

\(\displaystyle \frac{dy}{dt}= cos(t)\)

Substitute the knowns into the formula.

\(\displaystyle \frac{dx}{dy}=\frac{\frac{dx}{dt}}{\frac{dy}{dt}}=\frac{-sin(t)}{cos(t)}=-tan(t)\)

Example Question #7 : Derivatives Of Parametrics

Solve for \(\displaystyle \frac{dy}{dx}\) if \(\displaystyle x=5t+9\) and \(\displaystyle y=2t+7\).

Possible Answers:

\(\displaystyle -\frac{5}{2}\)

\(\displaystyle 0\)

\(\displaystyle \frac{2}{5}\)

\(\displaystyle \frac{5}{2}\)

\(\displaystyle -\frac{2}{5}\)

Correct answer:

\(\displaystyle \frac{2}{5}\)

Explanation:

We can determine that \(\displaystyle \frac{dy}{dx}=\frac{\frac{dy}{dt}}{\frac{dx}{dt}}\)  since the \(\displaystyle dt\) terms will cancel out in the division process.

Since \(\displaystyle x=5t+9\) and \(\displaystyle y=2t+7\),  we can use the Power Rule

\(\displaystyle \frac{d}{dx}x^{n}=nx^{n-1}\) for all \(\displaystyle n\neq0\) to derive

\(\displaystyle \frac{dx}{dt}=(1)5x^{1-1}=5\)and \(\displaystyle \frac{dy}{dt}=(1)2t^{1-1}+(0)7=2\).

Thus:

\(\displaystyle \frac{dy}{dx}=\frac{\frac{dy}{dt}}{\frac{dx}{dt}}=\frac{2}{5}\).

Example Question #2 : Derivatives Of Parametrics

Solve for \(\displaystyle \frac{dy}{dx}\) if \(\displaystyle x=2t-5\) and \(\displaystyle y=3t+8\).

Possible Answers:

None of the above

\(\displaystyle \frac{2}{3}\)

\(\displaystyle -\frac{3}{2}\)

\(\displaystyle \frac{3}{2}\)

\(\displaystyle -\frac{2}{3}\)

Correct answer:

\(\displaystyle \frac{3}{2}\)

Explanation:

We can determine that \(\displaystyle \frac{dy}{dx}=\frac{\frac{dy}{dt}}{\frac{dx}{dt}}\)  since the \(\displaystyle dt\) terms will cancel out in the division process.

Since \(\displaystyle x=2t-5\) and \(\displaystyle y=3t+8\), we can use the Power Rule

\(\displaystyle \frac{d}{dx}x^{n}=nx^{n-1}\) for all \(\displaystyle n\neq0\) to derive

\(\displaystyle \frac{dx}{dt}=(1)2t^{1-1}-(0)5=2\)and \(\displaystyle \frac{dy}{dt}=(1)3t^{1-1}+(0)8=3\) .

Thus:

\(\displaystyle \frac{dy}{dx}=\frac{\frac{dy}{dt}}{\frac{dx}{dt}}=\frac{3}{2}}\).

Example Question #121 : Parametric, Polar, And Vector

Solve for \(\displaystyle \frac{dy}{dx}\) if \(\displaystyle x=9t+1\) and \(\displaystyle y=4t+10\).

Possible Answers:

None of the above

\(\displaystyle \frac{9}{4}\)

\(\displaystyle -\frac{4}{9}\)

\(\displaystyle -\frac{9}{4}\)

\(\displaystyle \frac{4}{9}\)

Correct answer:

\(\displaystyle \frac{4}{9}\)

Explanation:

We can determine that \(\displaystyle \frac{dy}{dx}=\frac{\frac{dy}{dt}}{\frac{dx}{dt}}\)  since the \(\displaystyle dt\) terms will cancel out in the division process.

Since \(\displaystyle x=9t+1\) and \(\displaystyle y=4t+10\), we can use the Power Rule

\(\displaystyle \frac{d}{dx}x^{n}=nx^{n-1}\) for all \(\displaystyle n\neq0\) to derive 

\(\displaystyle \frac{dx}{dt}=(1)9t^{1-1}+(0)1=9\) and \(\displaystyle \frac{dy}{dt}=(1)4t^{1-1}+(0)10=4\)  .

Thus:

\(\displaystyle \frac{dy}{dx}=\frac{\frac{dy}{dt}}{\frac{dx}{dt}}=\frac{4}{9}}\).

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