If points A, D and C lie on a straight line, then angles ∠ADB and ∠BDC are supplementary angles and

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1. If you move point B right from the initial position, then m∠ADB increases and m∠BDC decreases, but

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2. If you move point B left from the initial position, then m∠ADB decreases and m∠BDC increases, but

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3. When point B lie on the line ADC, then:
a. Point B lies on the right hand from point D:

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b. Point B lies on the left hand from point D:

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4. When point B is reflected about the line ADC situation is the same as in parts 1 and 2.
Conclusion: 
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