Answer:
we can use two ways to write 180° along with the inscribed angle theorem to obtain the desired relation
Explanation:
Given ∆ABC inscribed in a circle O where E is the midpoint of arc BC and ED is a diameter, you want to prove ∠DEA = 1/2(∠B -∠C).
Setup
We can add add arcs to make 180° in two different ways, then equate the sums.
arc EB +arc BA +arc AD = 180°
arc EC +arc CA -arc AD = 180°
Equating these expressions for 180°, we have ...
arc EB +arc BA +arc AD = arc EC +arc CA -arc AD
Solution
Recognizing that arc EB = arc EC, we can subtract (arc EB +arc BA -arc AD) from both sides to get ...
2·arc AD = arc CA -arc BA
The inscribed angle theorem tells us ...
- arc AD = 2∠DEA
- arc CA = 2∠B
- arc BA = 2∠C
Making these substitutions into the above equation, we have ...
4∠DEA = 2∠B -2∠C
Dividing by 4 gives the relation we're trying to prove:
∠DEA = 1/2(∠B -∠C)