GMAT Math : Tangent Lines

Study concepts, example questions & explanations for GMAT Math

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Example Questions

Example Question #1 : Tangent Lines

Determine the equation of the line tangent to the curve  \displaystyle y=x^2  at the point  \displaystyle (-1,1)  ?

Possible Answers:

\displaystyle y=-2x-1

\displaystyle y=-x+2

\displaystyle y=-2x+1

\displaystyle y=x-2

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

Correct answer:

\displaystyle y=-2x-1

Explanation:

To find the equation of a line tangent to a curve at a certain point, we first need to find the slope of the curve at that point. To find the slope of a function at any point, we need its derivative:

\displaystyle y=x^2\rightarrow y'=2x

Now we can plug in the x value of the given point to find the slope of the function, and therefore the slope of tangent line, at that point:

\displaystyle y'=2x

\displaystyle y'(-1)=2(-1)=-2\rightarrow m=-2

Now that we have our slope, we can simply plug this value in with the given point to solve for the y intercept of the tangent line:

\displaystyle y=mx+b

\displaystyle 1=(-2)(-1)+b

\displaystyle b=-1

We have calculated the slope of the tangent line and its y intercept, so the equation for the line tangent to the curve  \displaystyle y=x^2  at the point  \displaystyle (-1,1)  in standard form is:

\displaystyle y=-2x-1

Example Question #1 : Tangent Lines

Determine the equation of the line tangent to the following curve at the point  \displaystyle (2,12).

\displaystyle f(x)=3x^2+4x-8

Possible Answers:

\displaystyle y=-10x+11

\displaystyle y=16x-20

\displaystyle y=10x-11

\displaystyle y=16x+13

\displaystyle y=-16x+20

Correct answer:

\displaystyle y=16x-20

Explanation:

First we find the slope of the tangent line by taking the derivative of the function and plugging in the \displaystyle x-value of the point where we want to know the slope:

\displaystyle f(x)=3x^2+4x-8

\displaystyle f'(x)=6x+4

\displaystyle f'(2)=6(2)+4=16

Now that we know the slope of the tangent line, we can plug it into the equation for a line along with the coordinates of the given point in order to calculate the \displaystyle y-intercept:

\displaystyle y=mx+b

\displaystyle 12=(16)(2)+b

\displaystyle b=-20

We now have \displaystyle m and \displaystyle b, so we can write the equation of the tangent line:

\displaystyle y=16x-20

Example Question #2 : Tangent Lines

Find the equation of a line tangent to the curve \displaystyle y=x^{2}-2x+1 at the point \displaystyle (3,4).

Possible Answers:

None of the above equations

\displaystyle y=4x+8

\displaystyle y=4x-4

\displaystyle y=4x-8

\displaystyle y=4x+4

Correct answer:

\displaystyle y=4x-8

Explanation:

To find the equation of a line tangent to the curve \displaystyle y=x^{2}-2x+1 at the point \displaystyle (3,4), we must first find the slope of the curve at the point \displaystyle (3,4) by solving the derivative \displaystyle y'=2x-2 at that point:

\displaystyle y'(3)=2(3)-2=6-2=4\rightarrow m=4

 

Given the slope, we can now plug the given point and the slope at that point into the slope-intercept form of the tangent line \displaystyle y=mx+b and solve for the \displaystyle y-intercept \displaystyle b:

\displaystyle (4)=(4)(3)+b

\displaystyle 4=12+b

\displaystyle -8=b

Given our slope, the chosen point, and the \displaystyle y-intercept, we have the equation of our tangent line:

\displaystyle y=4x-8

Example Question #3 : Calculating The Equation Of A Tangent Line

Find the equation of a line tangent to the curve \displaystyle y=2x^{2}+5x+2 at the point \displaystyle (2,-1).

Possible Answers:

\displaystyle y=27x+13

\displaystyle y=13x+27

\displaystyle y=27x-13

\displaystyle y=13x-27

None of the above

Correct answer:

\displaystyle y=13x-27

Explanation:

To find the equation of a line tangent to the curve \displaystyle y=2x^{2}+5x+2 at the point \displaystyle (2,-1), we must first find the slope of the curve at the point \displaystyle (2,-1) by solving the derivative \displaystyle y'=4x+5 at that point:

\displaystyle y'(2)=4(2)+5=8+5=13\rightarrow m=13

Given the slope, we can now plug the given point and the slope at that point into the slope-intercept form of the tangent line \displaystyle y=mx+b and solve for the \displaystyle y-intercept \displaystyle b:

\displaystyle (-1)=(13)(2)+b

\displaystyle -1=26+b

\displaystyle -27=b

Given our slope, the chosen point, and the \displaystyle y-intercept, we have the equation of our tangent line:

\displaystyle y=13x-27

Example Question #2 : Tangent Lines

Determine the equation of the tangent line to the following curve at the point  \displaystyle (2,-7) :

\displaystyle f(x)=-2x^2+9x-17

Possible Answers:

\displaystyle y=x-7

\displaystyle y=x+9

\displaystyle y=-2x+3

\displaystyle y=x-9

\displaystyle y=-2x-3

Correct answer:

\displaystyle y=x-9

Explanation:

First find the slope of the tangent line by taking the derivative of the function and plugging in the x value of the given point to find the slope of the curve at that location:

\displaystyle f(x)=-2x^2+9x-17

\displaystyle f'(x)=-4x+9

\displaystyle f'(2)=-4(2)+9=1

So the slope of the tangent line to the curve at the given point is  \displaystyle m=1.  The next step is to plug this slope into the formula for a line, along with the coordinates of the given point, to solve for the value of the y intercept of the tangent line:

\displaystyle y=mx+b

\displaystyle -7=1(2)+b\rightarrow b=-9

We now know the slope and y intercept of the tangent line, so we can write its equation as follows:

\displaystyle y=x-9

Example Question #4 : Tangent Lines

Find the equation of a line tangent to the curve \displaystyle y=4x^{2}-3x+7 at the point \displaystyle (-2,-3).

Possible Answers:

\displaystyle y=19x+41

None of the above

\displaystyle y=-19x+41

\displaystyle y=-19x-41

\displaystyle y=19x-41

Correct answer:

\displaystyle y=-19x-41

Explanation:

To find the equation of a line tangent to the curve \displaystyle y=4x^{2}-3x+7 at the point \displaystyle (-2,-3), we must first find the slope of the curve at the point \displaystyle (-2,-3) by solving the derivative \displaystyle y'=8x-3 at that point:

\displaystyle y'(-2)=8(-2)-3=-16-3=-19\rightarrow m=-19

Given the slope, we can now plug the given point and the slope at that point into the slope-intercept form of the tangent line \displaystyle y=mx+b and solve for the \displaystyle y-intercept \displaystyle b:

\displaystyle (-3)=(-19)(-2)+b

\displaystyle -3=38+b

\displaystyle -41=b

Given our slope, the chosen point, and the \displaystyle y-intercept, we have the equation of our tangent line:

\displaystyle y=-19x-41

Example Question #1 : Calculating The Slope Of A Tangent Line

Find the slope of the line tangent to \displaystyle f(x) at \displaystyle x=3.

\displaystyle \small f(x)=x^2-6x+9

Possible Answers:

\displaystyle 1

\displaystyle 0

\displaystyle -1

\displaystyle 2

\displaystyle -2

Correct answer:

\displaystyle 0

Explanation:

f(x) in this case gives us a parabola. If we factor the f(x) we get:

\displaystyle \small f(x)=(x-3)(x-3)

This equation has one zero and that is at x=3. This also means we have a minimum at x=3.

The slope of a line tangent to a minimum or maximum is always 0, so our slope is zero.

Example Question #2 : Calculating The Slope Of A Tangent Line

Calculate the slope of the tangent line to the following curve at the point  \displaystyle (1,4).

\displaystyle f(x)=-2x^2+13x-7

Possible Answers:

\displaystyle 5

\displaystyle 9

\displaystyle 11

\displaystyle 12

\displaystyle 7

Correct answer:

\displaystyle 9

Explanation:

The slope of the tangent line to a curve at any point is simply the slope of the curve at that point. To find the slope of the function at any point, we take the derivative:

\displaystyle f(x)=-2x^2+13x-7

\displaystyle f'(x)=-4x+13

Now we can plug in the x value of the given point, \displaystyle (1,4), which gives us the slope of the tangent line to the curve at that point:

\displaystyle f'(x)=-4x+13

\displaystyle f'(1)=-4(1)+13=9

Example Question #3 : Calculating The Slope Of A Tangent Line

Calculate the slope of the line tangent to the curve \displaystyle y=3x^2-5x+7 at the point  \displaystyle (2,15).

Possible Answers:

\displaystyle -5

\displaystyle -3

\displaystyle 9

\displaystyle 7

\displaystyle 10

Correct answer:

\displaystyle 7

Explanation:

To calculate the slope of a tangent line at a particular point, we need to know the slope of the curve at that point. In order to find the slope of a curve at any point, we need to calculate its derivative:

\displaystyle y=3x^2-5x+7

\displaystyle y'=6x-5

The derivative describes the slope of the curve at any point, so we need to plug in the \displaystyle x value of the given point  \displaystyle (2,15)  to find the slope of the tangent line at that location:

\displaystyle y'=6x-5

\displaystyle y'(2)=6(2)-5=7

Example Question #1 : Calculating The Slope Of A Tangent Line

Calculate the slope of the line tangent to the following curve at  \displaystyle x=1 :

\displaystyle f(x)=6x^5-7x^4+2x^3

Possible Answers:

\displaystyle 8

\displaystyle 6

\displaystyle 1

\displaystyle 30

\displaystyle 15

Correct answer:

\displaystyle 8

Explanation:

First find the derivative of the function, and then plug in the given x coordinate, which will give the slope of the function at that location. The slope of the tangent line to a curve at a given location is equal to the slope of the function at that location:

\displaystyle f(x)=6x^5-7x^4+2x^3

\displaystyle f'(x)=30x^4-28x^3+6x^2

\displaystyle f'(1)=30(1)^4-28(1)^3+6(1)^2=8

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