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The measures of two angles of a triangle are 55 and 82 degrees. Find the measure of the third angle.

Solution

Step 1. Read the problem. Draw the figure and label it with the given information. .
Step 2. Identify what you are looking for. the measure of the third angle in a triangle
Step 3. Name. Choose a variable to represent it. Let x = the measure of the angle.
Step 4. Translate.
Write the appropriate formula and substitute. m A + m B + m C = 180
Step 5. Solve the equation. 55 + 82 + x = 180 137 + x = 180 x = 43
Step 6. Check.

55 + 82 + 43 180 180 = 180
Step 7. Answer the question. The measure of the third angle is 43 degrees.
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The measures of two angles of a triangle are 31 and 128 degrees. Find the measure of the third angle.

21 degrees

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The measures of two angles of a triangle are 49 and 75 degrees. Find the measure of the third angle.

56 degrees

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The perimeter of a triangular garden is 24 feet. The lengths of two sides are four feet and nine feet. How long is the third side?

Solution

Step 1. Read the problem. Draw the figure and label it with the given information. .
.
Step 2. Identify what you are looking for. length of the third side of a triangle
Step 3. Name. Choose a variable to represent it. Let c = the third side.
Step 4. Translate.
Write the appropriate formula and substitute. .
Substitute in the given information. .
Step 5. Solve the equation. .
.
Step 6. Check.

P = a + b + c 24 4 + 9 + 11 24 = 24
Step 7. Answer the question. The third side is 11 feet long.
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The perimeter of a triangular garden is 48 feet. The lengths of two sides are 18 feet and 22 feet. How long is the third side?

8 feet

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The lengths of two sides of a triangular window are seven feet and five feet. The perimeter is 18 feet. How long is the third side?

6 feet

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The area of a triangular church window is 90 square meters. The base of the window is 15 meters. What is the window’s height?

Solution

Step 1. Read the problem. Draw the figure and label it with the given information. .
Area = 90 m 2
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.
Step 4. Translate.
Write the appropriate formula. .
Substitute in the given information. .
Step 5. Solve the equation. .
.
Step 6. Check.

A = 1 2 b h 90 1 2 15 12 90 = 90
Step 7. Answer the question. The height of the triangle is 12 meters.
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The area of a triangular painting is 126 square inches. The base is 18 inches. What is the height?

14 inches

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A triangular tent door has area 15 square feet. The height is five feet. What is the base?

6 feet

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The triangle properties we used so far apply to all triangles. Now we will look at one specific type of triangle—a right triangle. A right triangle has one 90 ° angle, which we usually mark with a small square in the corner.

A right triangle with the largest angle marked 90 degrees.

Right triangle

A right triangle has one 90 ° angle, which is often marked with a square at the vertex.

One angle of a right triangle measures 28 ° . What is the measure of the third angle?

Solution

Step 1. Read the problem. Draw the figure and label it with the given information. .
Step 2. Identify what you are looking for. the measure of an angle
Step 3. Name. Choose a variable to represent it. Let x = the measure of an angle.
Step 4. Translate. m A + m B + m C = 180
Write the appropriate formula and substitute. x + 90 + 28 = 180
Step 5. Solve the equation. x + 118 = 180 x = 62
Step 6. Check.

180 90 + 28 + 62 180 = 180
Step 7. Answer the question. The measure of the third angle is 62 ° .
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Source:  OpenStax, Elementary algebra. OpenStax CNX. Jan 18, 2017 Download for free at http://cnx.org/content/col12116/1.2
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