1.Prove that the bisectors of a pair of vertically opposite angles are in the same straight line.

2. If two straight lines intersect each other, provethat the ray opposite to the bisector of one of theangles thus formed bisects the vertically opposite angle.

3. Ifone four angles formed by two intersecting lines is a right angle, thenshow that each of the four angles is a right angle.

4.AB,CD and EF are three concurrentlines passing through thepointOsuch that OF bisects /_BOF = 35^{o}, find /_BOC and/_AOD.

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