Q.

In figure, O is the centre of the circle. Determine and  AOB if PA and PB are tangents and  ABP=75°.


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a

 ABP=52.5°  and  ABP=127.5°

b

 ABP=50.5°  and  ABP=127.5°

c

 ABP=52.5°  and  ABP=128.5°

d

 ABP=52.5°  and  ABP=120.5° 

answer is A.

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Detailed Solution

Given that,  APB = 75°.
Question ImagePA and PB are the tangents to the circle from P at A and B respectively and OA and OB are the radii of the circle.
Hence, OA _|_ PA and OB_|_ PB.
Now, OAP = OBP = 90°.
In quadrilateral AOBP,
Sum of all the angles of a quadrilateral is 360°.
ABP + OAP + OBP + AOB = 360°
75° + 90° + 90° + AOB = 360°
255° + AOB = 360°
AOB = 360° - 255°
AOB = 105°
We know that angle subtended by an arc at the centre is double the angle subtended by the same arc at any point on the circle. Consider the minor arc AB, then  AOB = 2AQB
AQB= 12AOB =  12×105°
AQB = 52.5°
Since AQBM is a cyclic quadrilateral,
Opposite angles of a cyclic quadrilateral are supplementary.
AMB + AQB = 180°
AMB + 52.5° = 180°
AMB = 180° - 52.5°
AMB = 127.5°
Thus, the value of  ABP=52.5°  and  ABP=127.5°.
Hence, option 1 is correct.
 
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