Big Ideas Math Geometry, 2014
BI
Big Ideas Math Geometry, 2014 View details
Chapter Test
Continue to next subchapter

Exercise 12 Page 59

Supplementary angles add up to 180^(∘) and complementary angles add up to 90^(∘).

Supplementary Angles: ∠ AFB and ∠ BFE, ∠ AFC and ∠ EFC, ∠ AFD and ∠ DFE
Complementary Angles: ∠ AFB and ∠ BFC, ∠ CFD and ∠ DFE
m∠ DFE = 63^(∘)
m∠ BFC = 51 ^(∘)
m∠ BFE = 141 ^(∘)

Practice makes perfect

Let's think about each of the given tasks one at a time, starting with finding all of the supplementary angles.

Supplementary Angles

Supplementary angles are any two angles whose sum is 180^(∘). Since ∠ AFE is 180^(∘), any division of it will create supplementary angles. Using FB as the divider, we get the following scenario.

This means that ∠ AFB and ∠ BFE are supplementary angles. We can also use FC as a divider.

This means that ∠ AFC and ∠ EFC are also supplementary angles. Finally, we can divide ∠ AFE using FD.

Therefore, ∠ AFD and ∠ DFE is our last pair of supplementary angles.

Complementary Angles

Complementary angles are any two angles whose sum is 90^(∘). Since ∠ AFE is a straight angle and ∠ CFE is a right angle, ∠ AFC is also a right angle measuring 90^(∘).

Any division of either of these will create complementary angles.

Looking at the left-hand side, we can see that ∠ AFB and ∠ BFC are complementary angles. Similarly, on the right-hand side, ∠ CFD and ∠ DFE are also complementary angles.

m∠ DFE

From the diagram, we know that m∠ CFD is 27^(∘).

Since ∠ CFD and ∠ DFE are complementary angles, the sum of their measures is 90^(∘).
m∠ CFD + m∠ DFE = 90^(∘)
27^(∘) + m∠ DFE = 90^(∘)
m∠ DFE = 63^(∘)
Therefore, m∠ DFE is 63^(∘).

m∠ BFC

We are given that m∠ AFB is 39^(∘).

Since ∠ AFB and ∠ BFC are complementary angles, their measures add up to 90^(∘).
m∠ AFB + m∠ BFC = 90^(∘)
39^(∘) + m∠ BFC = 90^(∘)
m∠ BFC = 51^(∘)
Hence, m∠ BFC is 51^(∘).

m∠ BFE

Finally, we know that ∠ AFB and ∠ BFE are supplementary angles.

That means that their measures sum to be 180^(∘).
m∠ AFB + m∠ BFE = 180^(∘)
39^(∘) + m∠ BFE = 180^(∘)
m∠ BFE = 141^(∘)
Our last angle, m∠ BFE, is 141^(∘).