Solve a Formula for a Specific Variable

We have all probably worked with some geometric formulas in our study of mathematics. Formulas are used in so many fields, it is important to recognize formulas and be able to manipulate them easily.

It is often helpful to solve a formula for a specific variable. If you need to put a formula in a spreadsheet, it is not unusual to have to solve it for a specific variable first. We isolate that variable on one side of the equals sign with a coefficient of one and all other variables and constants are on the other side of the equal sign.

Geometric formulas often need to be solved for another variable, too. The formula

V = \frac{1}{3} π r^{2} h

is used to find the volume of a right circular cone when given the radius of the base and height. In the next example, we will solve this formula for the height.

In the sciences, we often need to change temperature from Fahrenheit to Celsius or vice versa. If you travel in a foreign country, you may want to change the Celsius temperature to the more familiar Fahrenheit temperature.

Sometimes we might be given an equation that is solved for y and need to solve it for x, or vice versa. In the following example, we’re given an equation with both x and y on the same side and we’ll solve it for y.

Use Formulas to Solve Geometry Applications

In this objective we will use some common geometry formulas. We will adapt our problem solving strategy so that we can solve geometry applications. The geometry formula will name the variables and give us the equation to solve.

In addition, since these applications will all involve shapes of some sort, most people find it helpful to draw a figure and label it with the given information. We will include this in the first step of the problem solving strategy for geometry applications.

When we solve geometry applications, we often have to use some of the properties of the figures. We will review those properties as needed.

The next example involves the area of a triangle. The area of a triangle is one-half the base times the height. We can write this as

A = \frac{1}{2} b h,

In the next example, we will work with a right triangle. To solve for the measure of each angle, we need to use two triangle properties. In any triangle, the sum of the measures of the angles is

180 ° .

Also, since the triangle is a right triangle, we remember that a right triangle has one

90 °

Here, we will have to define one angle in terms of another. We will wait to draw the figure until we write expressions for all the angles we are looking for.

The next example uses another important geometry formula. The Pythagorean Theorem tells how the lengths of the three sides of a right triangle relate to each other. Writing the formula in every exercise and saying it aloud as you write it may help you memorize the Pythagorean Theorem.

The next example is about the perimeter of a rectangle. Since the perimeter is just the distance around the rectangle, we find the sum of the lengths of its four sides—the sum of two lengths and two widths. We can write is as

P = 2 L + 2 W

is the width. To solve the example, we will need to define the length in terms of the width.

The next example is about the perimeter of a triangle. Since the perimeter is just the distance around the triangle, we find the sum of the lengths of its three sides. We can write this as

P = a + b + c,

Applications of these geometric properties can be found in many everyday situations as shown in the next example.

Key Concepts

Practice Makes Perfect

Solve a Formula for a Specific Variable

In the following exercises, solve the given formula for the specified variable.

Solve the formula $C = π d$

for d.

d = \frac{C}{π}

Solve the formula $C = π d$

for $π .$

Solve the formula $V = L W H$

for L.

L = \frac{V}{W H}

Solve the formula $V = L W H$

for H.

Solve the formula $A = \frac{1}{2} b h$

for b.

b = \frac{2 A}{h}

Solve the formula $A = \frac{1}{2} b h$

for h.

Solve the formula* * *

A = \frac{1}{2} d_{1} d_{2}

for $d_{1} .$

d_{1} = \frac{2 A}{d_{2}}

Solve the formula* * *

A = \frac{1}{2} d_{1} d_{2}

for $d_{2} .$

Solve the formula* * *

A = \frac{1}{2} h (b_{1} + b_{2})

for $b_{1} .$

b_{1} = \frac{2 A}{h} - b_{2}

Solve the formula* * *

A = \frac{1}{2} h (b_{1} + b_{2})

for $b_{2} .$

Solve the formula* * *

h = 54 t + \frac{1}{2} a t^{2}

for a.

a = \frac{2 h - 108 t}{t^{2}}

Solve the formula* * *

h = 48 t + \frac{1}{2} a t^{2}

for a.

Solve $180 = a + b + c$

for a.

a = 180 - b - c

Solve $180 = a + b + c$

for c.

Solve the formula* * *

A = \frac{1}{2} p l + B

for p.

p = \frac{2 A - 2 B}{l}

Solve the formula* * *

A = \frac{1}{2} p l + B

for l.

Solve the formula* * *

P = 2 L + 2 W

for L.

L = \frac{P - 2 W}{2}

Solve the formula* * *

P = 2 L + 2 W

for W.

In the following exercises, solve for the formula for y.

Solve the formula* * *

8 x + y = 15

for y.

y = 15 - 8 x

Solve the formula* * *

9 x + y = 13

for y.

Solve the formula* * *

−4 x + y = −6

for y.

y = −6 + 4 x

Solve the formula* * *

−5 x + y = −1

for y.

Solve the formula* * *

x - y = −4

for y.

y = 4 + x

Solve the formula* * *

x - y = −3

for y.

Solve the formula* * *

4 x + 3 y = 7

for y.

y = \frac{7 - 4 x}{3}

Solve the formula* * *

3 x + 2 y = 11

for y.

Solve the formula* * *

2 x + 3 y = 12

for y.

y = \frac{12 - 2 x}{3}

Solve the formula* * *

5 x + 2 y = 10

for y.

Solve the formula* * *

3 x - 2 y = 18

for y.

y = \frac{18 - 3 x}{−2}

Solve the formula* * *

4 x - 3 y = 12

for y.

Use Formulas to Solve Geometry Applications

In the following exercises, solve using a geometry formula.

A triangular flag has area 0.75 square feet and height 1.5 foot. What is its base?

1 foot

A triangular window has area 24 square feet and height six feet. What is its base?

What is the base of a triangle with area 207 square inches and height 18 inches?

23 inches

What is the height of a triangle with area 893 square inches and base 38 inches?

The two smaller angles of a right triangle have equal measures. Find the measures of all three angles.

45 °, 45 °, 90 °

The measure of the smallest angle of a right triangle is $20 °$

less than the measure of the next larger angle. Find the measures of all three angles.

The angles in a triangle are such that one angle is twice the smallest angle, while the third angle is three times as large as the smallest angle. Find the measures of all three angles.

30 °, 60 °, 90 °

The angles in a triangle are such that one angle is 20 more than the smallest angle, while the third angle is three times as large as the smallest angle. Find the measures of all three angles.

In the following exercises, use the Pythagorean Theorem to find the length of the hypotenuse.

![The figure is a right triangle with sides 9 units and 12 units.](/algebra-intermediate-book/resources/CNX_IntAlg_Figure_02_03_201_img.jpg)

![The figure is a right triangle with sides 16 units and 12 units.](/algebra-intermediate-book/resources/CNX_IntAlg_Figure_02_03_202_img.jpg)

![The figure is a right triangle with sides 15 units and 20 units.](/algebra-intermediate-book/resources/CNX_IntAlg_Figure_02_03_203_img.jpg)

![The figure is a right triangle with sides 5 units and 12 units.](/algebra-intermediate-book/resources/CNX_IntAlg_Figure_02_03_204_img.jpg)

In the following exercises, use the Pythagorean Theorem to find the length of the leg. Round to the nearest tenth if necessary.

![The figure is a right triangle with sides 6 units and 10 units.](/algebra-intermediate-book/resources/CNX_IntAlg_Figure_02_03_205_img.jpg)

![The figure is a right triangle with a side that is 9 units and a hypotenuse that is 13 units.](/algebra-intermediate-book/resources/CNX_IntAlg_Figure_02_03_206_img.jpg)

![The figure is a right triangle with a side that is 5 units and a hypotenuse that is 13 units.](/algebra-intermediate-book/resources/CNX_IntAlg_Figure_02_03_207_img.jpg)

![The figure is a right triangle with a side that is 16 units and a hypotenuse that is 20 units.](/algebra-intermediate-book/resources/CNX_IntAlg_Figure_02_03_208_img.jpg)

![The figure is a right triangle with a side that is 8 units and a hypotenuse that is 13 units.](/algebra-intermediate-book/resources/CNX_IntAlg_Figure_02_03_209_img.jpg)

10.2

![The figure is a right triangle with sides that are both 6 units.](/algebra-intermediate-book/resources/CNX_IntAlg_Figure_02_03_210_img.jpg)

![The figure is a right triangle with sides that are 5 units and 11 units.](/algebra-intermediate-book/resources/CNX_IntAlg_Figure_02_03_211_img.jpg)

9.8

![The figure is a right triangle with sides that are 5 units and 7 units.](/algebra-intermediate-book/resources/CNX_IntAlg_Figure_02_03_212_img.jpg)

In the following exercises, solve using a geometry formula.

The width of a rectangle is seven meters less than the length. The perimeter is 58 meters. Find the length and width.

18 meters, 11 meters

The length of a rectangle is eight feet more than the width. The perimeter is 60 feet. Find the length and width.

The width of the rectangle is 0.7 meters less than the length. The perimeter of a rectangle is 52.6 meters. Find the dimensions of the rectangle.

13.5

m, $12.8$

The length of the rectangle is 1.1 meters less than the width. The perimeter of a rectangle is 49.4 meters. Find the dimensions of the rectangle.

The perimeter of a rectangle of 150 feet. The length of the rectangle is twice the width. Find the length and width of the rectangle.

25 ft, 50 ft

The length of the rectangle is three times the width. The perimeter of a rectangle is 72 feet. Find the length and width of the rectangle.

The length of the rectangle is three meters less than twice the width. The perimeter of a rectangle is 36 meters. Find the dimensions of the rectangle.

7 m, 11 m

The length of a rectangle is five inches more than twice the width. The perimeter is 34 inches. Find the length and width.

The perimeter of a triangle is 39 feet. One side of the triangle is one foot longer than the second side. The third side is two feet longer than the second side. Find the length of each side.

12 ft, 13 ft, 14 ft

The perimeter of a triangle is 35 feet. One side of the triangle is five feet longer than the second side. The third side is three feet longer than the second side. Find the length of each side.

One side of a triangle is twice the smallest side. The third side is five feet more than the shortest side. The perimeter is 17 feet. Find the lengths of all three sides.

3 ft, 6 ft, 8 ft

One side of a triangle is three times the smallest side. The third side is three feet more than the shortest side. The perimeter is 13 feet. Find the lengths of all three sides.

The perimeter of a rectangular field is 560 yards. The length is 40 yards more than the width. Find the length and width of the field.

120 yd, 160 yd

The perimeter of a rectangular atrium is 160 feet. The length is 16 feet more than the width. Find the length and width of the atrium.

A rectangular parking lot has perimeter 250 feet. The length is five feet more than twice the width. Find the length and width of the parking lot.

40 ft, 85 ft

A rectangular rug has perimeter 240 inches. The length is 12 inches more than twice the width. Find the length and width of the rug.

In the following exercises, solve. Approximate answers to the nearest tenth, if necessary.

A 13-foot string of lights will be attached to the top of a 12-foot pole for a holiday display as shown. How far from the base of the pole should the end of the string of lights be anchored?

The figure is an illustration that shows a 13 foot string of lights attached diagonally to the top of a 12 foot pole.

5 feet

am wants to put a banner across her garage door diagonally, as shown, to congratulate her son for his college graduation. The garage door is 12 feet high and 16 feet wide. Approximately how long should the banner be to fit the garage door?

The figure is an illustration of a banner positioned diagonally across a garage door that is 12 feet high and 16 feet wide.

Chi is planning to put a diagonal path of paving stones through her flower garden as shown. The flower garden is a square with side 10 feet. What will the length of the path be?

The figure is an illustration of a diagonal path of stones through a square garden with 10 foot sides.

14.1 feet

Brian borrowed a 20-foot extension ladder to use when he paints his house. If he sets the base of the ladder six feet from the house as shown, how far up will the top of the ladder reach?

The figure is an illustration of a house that has a ladder against it. The ladder is 20 feet. Its base is positioned 6 feet from the house.

Everyday Math

Converting temperature While on a tour in Greece, Tatyana saw that the temperature was 40° Celsius. Solve for F in the formula $C = \frac{5}{9} (F - 32)$

to find the Fahrenheit temperature.

104 ° F

Converting temperature Yon was visiting the United States and he saw that the temperature in Seattle one day was 50° Fahrenheit. Solve for C in the formula $F = \frac{9}{5} C + 32$

to find the Celsius temperature.

Christa wants to put a fence around her triangular flowerbed. The sides of the flowerbed are six feet, eight feet and 10 feet. How many feet of fencing will she need to enclose her flowerbed?

24 ft

Jose just removed the children’s play set from his back yard to make room for a rectangular garden. He wants to put a fence around the garden to keep the dog out. He has a 50-foot roll of fence in his garage that he plans to use. To fit in the backyard, the width of the garden must be 10 feet. How long can he make the other side?

Writing Exercises

If you need to put tile on your kitchen floor, do you need to know the perimeter or the area of the kitchen? Explain your reasoning.

Answers will vary.

If you need to put a fence around your backyard, do you need to know the perimeter or the area of the backyard? Explain your reasoning.

Look at the two figures below.

A figure of a rectangle with a width that is 2 units and a length that is 8 units and a square with sides that are 4 units. ⓐ Which figure looks like it has the larger area? Which looks like it has the larger perimeter?* * *

ⓑ Now calculate the area and perimeter of each figure. Which has the larger area? Which has the larger perimeter?* * *

ⓐ Answers will vary. ⓑ The areas are the same. The $2 \times 8$

rectangle has a larger perimeter than the $4 \times 4$

square.* * *

Write a geometry word problem that relates to your life experience, then solve it and explain all your steps.

Self Check

ⓐ After completing the exercises, use this checklist to evaluate your mastery of the objectives of this section.

This table has four columns and three rows. The first row is a header and it labels each column, “I can…”, “Confidently,” “With some help,” and “No-I don’t get it!” In row 2, the I can was solve a formula for a specific variable. In row 3, the I can was use formulas to solve geometry applications. ⓑ What does this checklist tell you about your mastery of this section? What steps will you take to improve?

Write the formula.
{: valign=”top”}	Remove the fraction on the right.
{: valign=”top”}	Simplify.
{: valign=”top”}	Divide both sides by $π r^{2} .$

from each side.
{: valign=”top”}	Simplify.
{: valign=”top”}	Solve for h by dividing both sides by $2 π r .$

Step 1. Read the problem.
Step 2. Identify what you are looking for.	height of a triangle
Step 3. Name.
Choose a variable to represent it.	Let $h =$ the height.
Draw the figure and label it with the given information.	Area = 126 sq. in.

Step 4. Translate.
Write the appropriate formula.	$A = \frac{1}{2} b h$
Substitute in the given information.	$126 = \frac{1}{2} \cdot 18 \cdot h$
Step 5. Solve the equation.	$126 = 9 h$
Divide both sides by 9.	$14 = h$
Step 6. Check. $\begin{matrix} A & = & \frac{1}{2} b h \\ 126 & \overset{?}{=} & \frac{1}{2} \cdot 18 \cdot 14 \\ 126 & = & 126 ✓ \end{matrix}$
Step 7. Answer the question.	The height of the triangle is 14 inches.

Step 1. Read the problem.
Step 2. Identify what you are looking for.	the measures of all three angles
Step 3. Name. Choose a variable to represent it.	$\begin{matrix} Let a & = & 1^{st} angle. \\ a + 40 & = & 2^{nd} angle \\ 90 & = & 3^{rd} angle (the right angle) \end{matrix}$
Draw the figure and label it with the given information.
Step 4. Translate.
Write the appropriate formula.
Substitute into the formula.
Step 5. Solve the equation.
Step 6. Check. $\begin{matrix} 25 + 65 + 90 & \overset{?}{=} & 180 \\ 180 & = & 180 ✓ \end{matrix}$
Step 7. Answer the question.	The three angles measure $25 °, 65 °,$ and $90 ° .$

Step 1. Read the problem.
Step 2. Identify what you are looking for.	the length of the leg of the triangle
Step 3. Name.
Choose a variable to represent it.	Let a = the leg of the triangle.
Label side a.
Step 4. Translate.
Write the appropriate formula. Substitute.	$\begin{matrix} a^{2} + b^{2} & = & c^{2} \\ a^{2} + 12^{2} & = & 13^{2} \end{matrix}$
Step 5. Solve the equation. Isolate the variable term. Use the definition of square root. Simplify.	$\begin{matrix} a^{2} + 144^{} & = & 169 \\ a^{2} & = & 25 \\ a & = & \sqrt{25} \\ a & = & 5 \end{matrix}$
Step 6. Check.
Step 7. Answer the question.	The length of the leg is 5.

Write the formula.
{: valign=”top”}	Isolate the h term by subtracting $2 π r^{2}$

We will isolate y on one side of the equation.
{: valign=”top”}	Subtract $6 x$

Step 1. Read the problem.
Step 2. Identify what we are looking for.	the length and the width
Step 3. Name. Choose a variable to represent the width. The length is six more than twice the width.	Let $w =$ width. $2 w + 6 =$ length $P = 96$ cm
Step 4. Translate.
Write the appropriate formula.
Substitute in the given information.
Step 5. Solve the equation.
Step 6. Check. $\begin{matrix} P & = & 2 L + 2 W \\ 96 & \overset{?}{=} & 2 \cdot 34 + 2 \cdot 14 \\ 96 & = & 96 ✓ \end{matrix}$
Step 7. Answer the question.	The length is 34 cm and the width is 14 cm.

Step 1. Read the problem.
Step 2. Identify what we are looking for.	the lengths of the three sides of a triangle
Step 3. Name. Choose a variable to represent the length of the first side.	$\begin{matrix} Let x & = & length of 1^{st} side. \\ x + 3 & = & length of 2^{nd} side \\ 2 x + 2 & = & length of 3^{rd} side \end{matrix}$
Step 4. Translate. Write the appropriate formula. Substitute in the given information.
Step 5. Solve the equation.
Step 6. Check. $29 \overset{?}{=} 6 + 9 + 14$ $29 = 29 ✓$
Step 7. Answer the question.	The lengths of the sides of the triangle are 6, 9, and 14 inches.

Step 1. Read the problem.
Step 2. Identify what we are looking for.	the length and width of the soccer field
Step 3. Name. Choose a variable to represent it. The length is 40 feet more than the width. Draw the figure and label it with the given information.	Let w = width. $w + 40 =$ length
Step 4. Translate. Write the appropriate formula and substitute.
Step 5. Solve the equation.
Step 6. Check. $\begin{matrix} P & = & 2 L + 2 W \\ 360 & \overset{?}{=} & 2 (110) + 2 (70) \\ 360 & = & 360 ✓ \end{matrix}$
Step 7. Answer the question.	The length of the soccer field is 110 feet and the width is 70 feet.

Step 1. Read the problem.
Step 2. Identify what we are looking for.	the distance from the corner that the bracket should be attached
Step 3. Name. Choose a variable to represent it. Draw the figure and label it with the given information.	Let $x =$ the distance from the corner.
Step 4. Translate. Write the appropriate formula and substitute.	$a^{2} + b^{2} = c^{2}$ $x^{2} + x^{2} = 10^{2}$
Step 5. Solve the equation. Isolate the variable. Use the definition of square root. Simplify. Approximate to the nearest tenth.	$\begin{matrix} 2 x^{2} & = & 100 \\ x^{2} & = & 50 \\ x & = & \sqrt{50} \\ x & \approx & 7.1 \end{matrix}$
Step 6. Check. $\begin{matrix} a^{2} + b^{2} & = & c^{2} \\ {(7.1)}^{2} + {(7.1)}^{2} & \approx & 10^{2} Yes. \end{matrix}$
Step 7. Answer the question.	Kelvin should fasten each piece of wood approximately 7.1” from the corner.