What is the measure of angle 5?

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Angle Pairs

Definitions

• Two angles are supplementary if the sum of their measures is 180

  
  .

• Two angles are complementary if the sum of their measures is 90

  
  .

• You can write "the measure of angle 1" as m∠1.

Example

Finding an Angle Measure

∠1 and ∠2 are complementary, and m∠2 = 32

Solution

• m∠1 + 32
  
   = 90
  
             Substitute 32
  
   for m∠2. 
• m∠1 = 58
  
                      Subtract 32
  
   from each side.

• m∠1 + m∠ 2 = 90

  
       Definition of complementary angles 

Practice

Are ∠1 and ∠2 are complementary, supplementary, or neither?

1.  m∠1 = 79

3.  m∠1 = 95

Vertical Angles

When two lines intersect at a point, they form two pairs of angles that do not share a side. These pairs are called vertical angles, and they always have the same measure.

∠1 and ∠3 are vertical angles.
m∠1 = m∠3

∠2 and ∠4 are vertical angles.
m∠2 = m∠4

Using Vertical Angles

Example

Find m∠2, m∠3, and m∠4.                 

Solution 

The diagram shows that m∠1 = 90

m∠1 + m∠2 = 180

∠2 and ∠4 are vertical angles. Their measures are equal, so m∠4 = 90

Perpendicular Lines

When two lines intersect to form one right angle, they form four right angles. Two lines that intersect at a right angle are called perpendicular lines.

Practice

Find the measures of the numbered angles.

5.       

6.   

Parallel Lines

Two lines in the same plane that do not intersect are called parallel lines. When a line intersects two parallel lines, several pairs of angles that are formed have equal measures.

Angles and Parallel Lines

Example

Corresponding Angles:   m∠1 = m∠5;  m∠2 = m∠6; 
                                           m∠3 = m∠7;  m∠4 = m∠8

Alternate Interior Angles: m∠3 = m∠6;  m∠4 = m∠5 

Alternate Exterior Angles: m∠1 = m∠8;  m∠2 = m∠7

Using Parallel Lines

Example

Use the diagram to find m∠1.     

Solution∠1 and ∠5 are corresponding angles, so they have equal measures. 

Find m∠5. The angle with measure 125

m∠5 + 125

∠1 and ∠5 have equal measures.

ANSWER: m∠1 = 55

Practice

Find the angle measure.   

7.  m∠2

8.  m∠3

9.  m∠4

10.  m∠6

Complete the statement.

11.  The sum of the measures of two _?_ angles is 180

12.  Two lines that intersect to form a right angle are called _?_.

Are the angles are complementary, supplementary, or neither?

13.  m∠1 = 62

14.  m∠1 = 51

Describe and correct the error in the solution.

15.   

m∠2 = 68

Find the value of the variable and the angle measures.

16.  m∠1= (5x + 15)

17.  m∠4 = (7n + 39)

18.   Find the angle measures in the weaving if m∠1 = 122

19.   A student designed the stationery border shown here. Explain how to find m∠2 if m∠1 = 135

Answers

1.  supplementary

2.  neither

3.  supplementary

4.  complementary

5.  m∠9 = 126o; m∠10 = 54o; m∠11 = 126o

6.  m∠6 = 43o; m∠7 = 137o; m∠8 = 43o

7.  85o

8.  95o

9.  85o

10.  85o

11.  supplementary

12.  perpendicular

13.  supplementary

14.  complementary

15.  Vertical angles are congruent, so m∠2 = 112o.

16.  5

17.  13

18.  m∠2 = 58o; m∠3 = 122o; m∠4 = 58o

19.  Angles 1 and 2 form a linear pair and are supplementary.  So to find m∠2, subtract m∠1 from 180o.  m∠2 = 180o - 135o = 45o.