Calculus 1 : How to find equation of line by graphing functions

Study concepts, example questions & explanations for Calculus 1

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Example Questions

Example Question #1 : Lines

What is the equation of the line tangent to f(x) = 4x3 – 2x2 + 4 at x = 5?

Possible Answers:

y = 280x – 946

None of the other answers

y = 220x – 550

y = 85x + 24

y = 44x + 245

Correct answer:

y = 280x – 946

Explanation:

First, take the derivative of f(x). This is very easy:

f'(x) = 12x2 – 4x

Now, the slope of the tangent line at x = 5 is f'(5). Evaluated, this is: f'(5) = 12 * 5 * 5 – 4 * 5 = 300 – 20 = 280

Now, we must find the intersection point on the original line:

f(5) = 4 * 53 – 2 * 52 + 4 = 500 – 50 + 4 = 454

Therefore, the point of tangent intersection is (5,454).

Using the point-slope form of linear equations, we can find the line:

(y – 454) = 280(x – 5)

y – 454 = 280x – 1400

y = 280x – 946

Example Question #1 : Lines

Find the equation of the line tangent to \displaystyle y=x^2-x+3 at the point \displaystyle (1,3).

Possible Answers:

\displaystyle y=2x+2

\displaystyle y=x+2

\displaystyle y=0.5x+1

\displaystyle y=0.5x+2

Correct answer:

\displaystyle y=x+2

Explanation:

The equation of the tangent line will have the form \displaystyle y-y_{0}=m(x-x_{0}), where \displaystyle m is the slope of the line and \displaystyle (x_{0},y_{0})=(1,3).  

To find the slope, we need to evaluate the derivative at \displaystyle x=1:

 \displaystyle \frac{dy}{dx}=\frac{d(x^2-x+3)}{dx}=2x-1\Rightarrow m=f'(1)=2*1-1=1

Now that we have the slope, we can determine the equation of the tangent line using the point-slope formula:

 \displaystyle y-y_{0}=m(x-x_{0})\Rightarrow y-3=1(x-1)\Rightarrow y=x+2.

Example Question #1 : How To Find Equation Of Line By Graphing Functions

Find the equation of the line tangent to \displaystyle f(x) at \displaystyle x=0.

\displaystyle f(x)=e^{3x} +x^2

Possible Answers:

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

\displaystyle y=3x+2

\displaystyle y=3x+1

\displaystyle y=3

\displaystyle y=x^2 +2

Correct answer:

\displaystyle y=3x+1

Explanation:

To find the equation of the line at that point, you need two things: the slope at that point and the y adjustment of the function, in the form \displaystyle y=mx+b, where m is the slope and \displaystyle b is the y adjustment. To get the slope, find the derivative of \displaystyle f(x) and plug in the desired point \displaystyle 0 for \displaystyle x, giving us an answer of \displaystyle 3 for the slope.

\displaystyle f'(x)=3e^{3x} + 2x

\displaystyle f'(0)=3

To find the y adjustment pick a point 0 in the original\displaystyle f(x) function. For simplicity, let's plug in \displaystyle x=0, which gives us a y of 1, so an easy point is \displaystyle (0,1). Next plug in those values into the equation of a line, \displaystyle y=mx + b. The new equation with all parameters plugged in is

\displaystyle 1=(3)(0)+b

Now you simply solve for \displaystyle b, which is \displaystyle 1.

Final equation of the line tangent to \displaystyle f(x) at \displaystyle x=0 is \displaystyle y=3x+1

Example Question #1 : Equation Of Line

Find the equation of the line tangent to \displaystyle f(x) at \displaystyle x=2.

\displaystyle f(x)= ln(2x+1) +3

Possible Answers:

\displaystyle y=\frac{2}{5}x +3

\displaystyle y=\frac{2}{3}x

\displaystyle y=3x

\displaystyle y=2x +3

\displaystyle y=\frac{2}{5}x

Correct answer:

\displaystyle y=\frac{2}{5}x +3

Explanation:

To get the slope, find the derivative of \displaystyle f(x) and plug in the desired point \displaystyle 2 for \displaystyle x, giving us an answer of \displaystyle \frac{2}{5} for the slope.

\displaystyle f'(x)=\frac{2}{2x+1}

\displaystyle f'(2)=\frac{2}{5}

Remember that the derivative of \displaystyle ln(u) = \frac{u'}{u}.

 

To find the \displaystyle y adjustment pick a point \displaystyle 0 (for example) in the original \displaystyle f(x) function. For simplicity, let's plug in \displaystyle x=0, which gives us a \displaystyle y of \displaystyle 3, so an easy point is \displaystyle (0,3). Next plug in those values into the equation of a line, \displaystyle y=mx + b. The new equation with all parameters plugged in is

\displaystyle 3=\frac{2}{5}(0)+b

The coefficient in front of the \displaystyle 0 is the slope.

Now you simply solve for \displaystyle b, which is \displaystyle 3.

Final equation of the line tangent to \displaystyle f(x) at\displaystyle x=2 is \displaystyle y=\frac{2}{5}x +3.

Example Question #2 : How To Find Equation Of Line By Graphing Functions

Find the equation of the line tangent to \displaystyle y=sin(2x) at \displaystyle x= \displaystyle \frac{\pi }{2}.

Possible Answers:

\displaystyle y=2x-\pi

\displaystyle y=2cos(2x)

\displaystyle y=-2x+1

\displaystyle y=-2x+\pi

\displaystyle y=0

Correct answer:

\displaystyle y=-2x+\pi

Explanation:

The equation of the tangent line is \displaystyle y=mx+b. To find \displaystyle m, the slope, calculate the derivative and plug in the desired point.

\displaystyle y'=2cos(2x)

\displaystyle y'=2cos\left(2\cdot \frac{\pi}{2}\right)

\displaystyle y'=2cos(\pi)

\displaystyle y'\left(\frac{\pi}{2} \right )=-2

The next step is to choose a coordinate on the original \displaystyle y function. We can choose any \displaystyle x value and calculate its \displaystyle y value.

Let's choose \displaystyle \frac{\pi }{2}.

\displaystyle y=sin\left(2\cdot \frac{\pi}{2}\right)=sin(\pi)=0

The \displaystyle y value at this point is \displaystyle 0.

Plugging in those values we can solve for \displaystyle b.

\displaystyle 0=(-2)\left(\frac{\pi }{2}\right)+b

Solving for \displaystyle b we get \displaystyle b=\displaystyle \pi.

\displaystyle y=-2x+\pi

Example Question #1 : How To Find Equation Of Line By Graphing Functions

A function, \displaystyle f, is given by

\displaystyle f(x)=x^2-sin(x).

Find the line tangent to \displaystyle f at \displaystyle x=\frac{\pi}{2}.

Possible Answers:

\displaystyle y=\pi x-\frac{\pi^2}{4}-1

\displaystyle y=2\pi x-\frac{\pi^2}{2}+1

\displaystyle y=\pi x-1

\displaystyle y=\pi x-\frac{\pi^2}{2}

\displaystyle y=\frac{\pi}{2} x-\frac{\pi^2}{4}

Correct answer:

\displaystyle y=\pi x-\frac{\pi^2}{4}-1

Explanation:

First we need to find the slope of \displaystyle f at \displaystyle x=\frac{\pi}{2}. To do this we need the derivative of \displaystyle f. To take the derivative we need to use the power rule for the first term and recognize that the derivative of sine is cosine.

\displaystyle \frac{df}{dx}=2x-cos(x)

At,

 \displaystyle x=\frac{\pi}{2}, \frac{df}{dx}=\pi

Now we need to know

\displaystyle f\left(\frac{\pi}{2}\right)=\frac{\pi^2}{4}-1.

Now we have a slope, \displaystyle m=\pi and a point \displaystyle (x_1,y_1)=\left(\frac{\pi}{2},\frac{\pi^2}{4}-1\right)

so we can use the point-slope formula to find the equation of the line.

\displaystyle y-y_1=m(x-x_1)

Plugging in and rearranging we find

\displaystyle y=\pi x-\frac{\pi^2}{4}-1.

Example Question #4 : How To Find Equation Of Line By Graphing Functions

Let \displaystyle f(x)=2x^4+e^x-ex

Find the equation for a line tangent to \displaystyle f(x) when \displaystyle x=1.

Possible Answers:

\displaystyle y-1=2(x-2)

\displaystyle y+8=2(x+1)

\displaystyle y-8=2(x-1)

\displaystyle y-2=8(x-1)

\displaystyle 8y=2(x-1)

Correct answer:

\displaystyle y-2=8(x-1)

Explanation:

First, evaluate \displaystyle f(x) when \displaystyle x=1.

\displaystyle f(1)=2(1)^4+e^1-e(1)=2+0\textbf{=2}

Thus, we need a line that contains the point \displaystyle (1,2)

Next, find the derivative of \displaystyle f(x) and evaluate it at \displaystyle x=1.

To find the derivative we will use the power rule,

\displaystyle f(x)=x^n \rightarrow f'(x)=nx^{n-1}.

\displaystyle f'(x)=8x^3+e^x-e

\displaystyle f'(1)=8(1)^3+e^1-e=8+0=8

This indicates that we need a line with a slope of 8.

In point-slope form, \displaystyle y-y_{1}=m(x-x_{1}), a line with the point \displaystyle (1,2) and a slope of 8 will be:

\displaystyle y-2=8(x-1)

Example Question #2 : Lines

What is the equation of the line tangent to \displaystyle f(x)=\arctan (1-x^3) at \displaystyle x=-1? Round to the nearest hundreth. 

Possible Answers:

\displaystyle y=-0.6

\displaystyle y=-0.6x

\displaystyle y=0.2x+1.31

\displaystyle y=0.2x

\displaystyle y=-0.6x+0.51

Correct answer:

\displaystyle y=-0.6x+0.51

Explanation:

The tangent line to \displaystyle f(x) at \displaystyle x=-1 must have the same slope as \displaystyle f(x).

Applying the chain rule we get

\displaystyle f'(x)=\frac{1}{1+(1-x^3)^2}(-3x^2).

Therefore the slope of the line is, 

\displaystyle f'(-1)=-\frac{3}{5}=-0.6.

In addition, the tangent line touches the graph of \displaystyle f(x) at \displaystyle x=-1. Since \displaystyle f(-1)=1.11, the point \displaystyle (-1, 1.11) lies on the line.

Plugging in the slope and point we get \displaystyle y=-0.6x+0.51.

Example Question #2 : Equation Of Line

Find the equation of the tangent line, where

\displaystyle f(x)=x^4-x^2+x-5, at \displaystyle x=1.

Possible Answers:

\displaystyle y=3x-1

\displaystyle y=x

\displaystyle y=3x+1

\displaystyle y=3x-7

\displaystyle y=3x+7

Correct answer:

\displaystyle y=3x-7

Explanation:

In order to find the equation of the tangent line at \displaystyle x=1, we first find the slope.

To do this we need to find \displaystyle f'(x).

\displaystyle f(x)=x^4-x^2+x-5

\displaystyle f'(x)=4x^3-2x+1

Since we have found \displaystyle f'(x), now we simply plug in 1.

\displaystyle f'(1)=4(1)^3-2(1)+1=3

Now we need to plug in 1, into \displaystyle f(x), to find a point that the tangent line touches.

\displaystyle f(1)=(1)^4-(1)^2+1-5=-4

Now we can use point-slope form to figure out what the equation of the tangent line is at \displaystyle x=1.

Remember that point-slope for is

\displaystyle y-y_0=m(x-x_0)

where \displaystyle x_0 and \displaystyle y_0 is the point where the tangent line touches \displaystyle f(x), and \displaystyle m is the slope of the tangent line.

In our case, \displaystyle x_0=1\displaystyle y_0=-4, and \displaystyle m=3.

\displaystyle y+4=3(x-1)

\displaystyle y+4=3x-3

Thus our tangent line equation at \displaystyle x=1 is

\displaystyle y=3x-7.

 

Example Question #3 : Equation Of Line

Find the equation of the tangent line of 

\displaystyle f(x)=x^3+x, at \displaystyle x=1.

Possible Answers:

\displaystyle y=2x+7

\displaystyle y=4x-2

\displaystyle y=4x+2

\displaystyle y=4x

\displaystyle y=-4x-2

Correct answer:

\displaystyle y=4x-2

Explanation:

In order to find the equation of the tangent line at \displaystyle x=1, we first find the slope.

To do this we need to find \displaystyle f'(x) using the power rule \displaystyle f(x)=x^n \rightarrow f'(x)=nx^{n-1}.

\displaystyle f(x)=x^3+x

\displaystyle f'(x)=3x^2+1

Since we have found \displaystyle f'(x), now we simply plug in 1.

\displaystyle f'(1)=3(1)^2+1=4

Now we need to plug in 1, into \displaystyle f(x), to find a point that the tangent line touches.

\displaystyle f(1)=(1)^3+1=2

Now we can use point-slope form to figure out what the equation of the tangent line is at \displaystyle x=1.

Remember that point-slope for is

\displaystyle y-y_0=m(x-x_0)

where \displaystyle x_0 and \displaystyle y_0 is the point where the tangent line touches \displaystyle f(x), and \displaystyle m is the slope of the tangent line.

In our case, \displaystyle x_0=1\displaystyle y_0=2, and \displaystyle m=4.

\displaystyle y-2=4(x-1)

\displaystyle y-2=4x-4

Thus our tangent line equation at \displaystyle x=1 is

\displaystyle y=4x-2.

 

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