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If A, B, C are three points on a circle with center O such that ∠AOB = 90° and ∠BOC = 120°, then ∠ABC =?
  • a)
    60°
  • b)
    90°
  • c)
    135°
  • d)
    75°
Correct answer is option 'D'. Can you explain this answer?
Most Upvoted Answer
If A, B, C are three points on a circle with center O such that ∠A...

Reflex ∠AOC = 360° – (90° + 120°) = 150° 
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Community Answer
If A, B, C are three points on a circle with center O such that ∠A...
Understanding the Circle Geometry
To find the angle ABC formed by points A, B, and C on a circle with center O, we start by analyzing the given information about the angles at the center of the circle.
Given Angles
- Angle AOB = 90 degrees
- Angle BOC = 120 degrees
Finding Angle AOC
- First, we can find angle AOC by adding angles AOB and BOC:
- Angle AOC = Angle AOB + Angle BOC
- Angle AOC = 90 degrees + 120 degrees = 210 degrees
Understanding Central and Inscribed Angles
- The angle ABC is an inscribed angle, which is half the measure of the central angle AOC that subtends the same arc AC.
- Therefore, to find angle ABC:
- Angle ABC = 1/2 * Angle AOC
Calculating Angle ABC
- Plugging in the value we found for angle AOC:
- Angle ABC = 1/2 * 210 degrees = 105 degrees
Finding the Correct Answer
- However, angle ABC is actually subtended by arc AC, and we need to consider the other angle formed at point B.
Final Calculation
- Since the angle ABC can also be calculated as:
- Angle ABC = 180 degrees - 105 degrees = 75 degrees
Thus, the correct answer is option (d) 75 degrees.
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