Advanced Mathematics Primary 5 Angles

Three straight lines meet at point O. On one side of line AA', the angles formed are angle AOB = 3x° and angle BOC = 2x°, where B and C are on two other lines. The angle COA' is 75°. Find angle A'OB.

A 63°
B 105°
C 117°
D 138°
Show Worked Solution

Worked Solution

Step 1: Angles AOB, BOC and COA' lie on one side of line AA' and together form a straight line. 3x + 2x + 75 = 180 5x = 105 x = 21 Step 2: Angle AOB = 3 × 21 = 63° Step 3: Angle A'OB and angle AOB are supplementary (angles on a straight line). Angle A'OB = 180° − 63° = 117°

Correct answer: 117°

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