You solved equations by adding or subtracting.(Lesson 4–3)

• Find the missing angle measure of a triangle.

• Classify triangles by properties and attributes.

• line segment

• triangle

• vertex

• congruent

Find Angle Measures

100 + 33 + x = 180 Substitution

133 + x = 180 Simplify.

133 – 133 + x = 180 – 133 Subtract 133 from each side.

x = 47 Simplify.

Answer: x is 47, so mF = 47°.

Find the value of x in ΔDEF.

Write an equation.

A. 57

B. 123

C. 139

D. 303

Find the value of x in ΔMNO.

Use Algebra to Find Angle Measures

ALGEBRA The measures of the angles of a triangle are 2x, x + 25, and x – 5. What are the measures of the angles?

4x +20 = 180Combine like terms.

x = 40 Simplify.

2x + (x +25) + (x + -5) = 180Write the equation.

The sum of the angle measures is 180.

- 20 -20 Subtract.

Divide each side by 4.

4x =

160 Simplify.

Answer: The measures of the angles are 80°, 65°, and 35°.

Substitute 40 for x in each expression.

2x = 2(40) = 80

x + 25 = 40 + 25 = 65

x - 5 = 40 - 5 = 35

Use Algebra to Find Angle Measures

Classify Triangles

A. Classify the triangle below by its angles and by its sides.

Answer: The triangle is an obtuse scalene triangle.

Angles: The triangle has an obtuse angle.

Sides: The triangle has no congruent sides.

Classify Triangles

B. Classify the triangle by its angles and by its sides.

Answer: The triangle is a right isosceles triangle.

Angles: The triangle has a right angle.

Sides: The triangle has two congruent sides.

A. right scalene

B. obtuse scalene

C. obtuse isoceles

D. acute equilateral

A. Classify the triangle by its angles and by its sides.

A. right isoceles

B. obtuse isoceles

C. obtuse scalene

D. acute equilateral

B. Classify the triangle by its angles and by its sides.