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Prove the vertical angle theorem

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Q. Prove the vertical angle theorem
  1. Identify Vertical Angles: Identify the vertical angles in a pair of intersecting lines.\newlineWhen two lines intersect, they form two pairs of opposite, or vertical, angles. These angles are called vertical angles.
  2. State Theorem: State the Vertical Angle Theorem.\newlineThe Vertical Angle Theorem states that the vertical angles formed by two intersecting lines are congruent, meaning they have equal measures.
  3. Set Up Proof: Set up a proof using arbitrary angles.\newlineLet's denote the intersecting lines as line ABAB and line CDCD, intersecting at point OO. The vertical angles formed are AOC\angle AOC and BOD\angle BOD, and the adjacent angles are AOB\angle AOB and COD\angle COD.
  4. Use Angle Sum Property: Use the fact that the sum of angles around a point is 360360 degrees.\newlineAt point OO, the sum of all angles around it is 360360 degrees. Therefore, AOC+AOB+BOD+COD=360\angle AOC + \angle AOB + \angle BOD + \angle COD = 360 degrees.
  5. Express Adjacent Angles: Express adjacent angles as supplementary. Since AOC\angle AOC and AOB\angle AOB are adjacent, they are supplementary, which means AOC+AOB=180\angle AOC + \angle AOB = 180 degrees. Similarly, BOD\angle BOD and COD\angle COD are supplementary, so BOD+COD=180\angle BOD + \angle COD = 180 degrees.
  6. Substitute Supplementary Sums: Substitute the supplementary angle sums into the total sum around point O.\newlineReplace AOC+AOB\angle AOC + \angle AOB with 180180 degrees and BOD+COD\angle BOD + \angle COD with 180180 degrees in the equation from step 44. This gives us 180180 degrees ++ 180180 degrees =360= 360 degrees, which is true.
  7. Conclude Angle Equality: Conclude that the vertical angles are equal.\newlineSince AOC+AOB=180\angle AOC + \angle AOB = 180 degrees and BOD+COD=180\angle BOD + \angle COD = 180 degrees, and we know that AOB=COD\angle AOB = \angle COD (as they are also vertical angles), we can deduce that AOC=BOD\angle AOC = \angle BOD.

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