Answer:
A.1: ∠BAC ≅ ∠BDC ≅ ∠EDF, ∠ACD ≅ ∠ABD ≅ ∠BDE ≅ ∠CDF
A.2: ∠1 ≅ ∠4, ∠2 ≅ ∠3 ≅ ∠5 ≅ ∠6
A.3: ∠2 ≅ ∠3
B.1: ∠ACD ≅ ∠CAB, ∠CDA ≅ ∠ABC, ∠DAC ≅ ∠BCA
B.2: ∠1 ≅ ∠3 ≅ ∠5, ∠2 ≅ ∠4 ≅ ∠6
see "additional comment" regarding listing pairs
Step-by-step explanation:
There are a number of ways angles can be identified as congruent. In each case, the converse of the proposition is also true.
- opposite angles of a parallelogram are congruent
- corresponding angles where a transversal crosses parallel lines are congruent
- alternate interior angles where a transversal crosses parallel lines are congruent
- vertical angles are congruent
- any two angles with the same measure are congruent
In these exercises, pairs of angles need to be examined to see which of these relations may apply.
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<h3>A</h3>
<u>Left</u>
ABCD is a parallelogram, so the congruent angles are opposite angles and any that are vertical or corresponding:
∠BAC ≅ ∠BDC ≅ ∠EDF ≅ 110° (3 pairs)
∠ACD ≅ ∠ABD ≅ ∠BDE ≅ ∠CDF ≅ 70° (6 pairs)
<u>Center</u>
∠1 ≅ ∠4 ≅ 66° (1 pair) . . . . vertical angles
∠2 ≅ ∠3 ≅ ∠5 ≅ ∠6 ≅ 57° (6 pairs) . . . . marked with the same measure, and their vertical angles
<u>Right</u>
Assuming that lines appearing to go in the same direction actually do go in the same direction, the only pair of congruent angles in the figure is ...
∠2 ≅ ∠3
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<h3>B</h3>
<u>Left</u>
Corresponding angles in congruent triangles are congruent. Here, the congruent triangles are ΔACD ≅ ΔCAB. So, the pairs of congruent angles are ...
∠ACD ≅ ∠CAB (30°)
∠CDA ≅ ∠ABC (90°)
∠DAC ≅ ∠BCA (60°)
<u>Right</u>
The corresponding angles and any vertical angles are congruent. This means all the odd-numbered angles in the figure are congruent, and all the even-numbered angles in the figure are congruent. The marked 72° angles show the "horizontal" segments are parallel by the converse of the corresponding angles theorem.
∠1 ≅ ∠3 ≅ ∠5 (72°) (3 pairs)
∠2 ≅ ∠4 ≅ ∠6 (108°) (3 pairs)
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<em>Additional comment</em>
The question asks you to list pairs of congruent angles. When 3 things are congruent, they can be arranged in 3 pairs:
a ≅ b ≅ c ⇒ (a≅b), (a≅c), (b≅c)
Similarly, when 4 things are congruent, they can be arranged in 6 pairs:
a ≅ b ≅ c ≅ d ⇒ (a≅b), (a≅c), (a≅d), (b≅c), (b≅d), (c≅d)
In the above, we have elected not to list all of the pairs, but to list the set of congruences from which pairs can be chosen.
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