Question : AB is a chord in the minor segment of a circle with centre O. C is a point on the minor arc (between A and B ). The tangents to the circle at A and B meet at a point P. If $\angle \mathrm{ACB}=108^{\circ}$, then $\angle \mathrm{APB}$ is equal to:
Option 1: $36^{\circ}$
Option 2: $54^{\circ}$
Option 3: $27^{\circ}$
Option 4: $18^{\circ}$
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Correct Answer: $36^{\circ}$
Solution :
Point D is taken on the major arc of the circle. Then, AD, BD, CB, and AC are joined.$\angle ACB = 108^\circ$ Now, $\angle ADB = (180 - 108)^\circ = 72^\circ$ Now, $\angle AOB = (72\times 2)^\circ = 144^\circ$ Since $OA = OB,\triangle AOB$ is an isosceles triangle and $\angle OBA = \angle OAB$. Now, $\angle AOB + \angle OBA + \angle OAB = 180^\circ$ ⇒ $2 \times \angle OBA = (180 - 144)^\circ$ ⇒ $\angle OBA = 18^\circ$ Now, $\angle OBA = \angle OAB = 18^\circ$ Now, $\angle OAP = \angle OAB + \angle BAP$ ⇒ $90^\circ = 18^\circ + \angle BAP$ ⇒ $\angle BAP = 72^\circ$ Now, $\angle OBP = \angle OBA + \angle ABP$ ⇒ $90^\circ = 18^\circ + \angle ABP$ ⇒ $\angle ABP = 72^\circ$ Now, $\angle APB + \angle ABP + \angle BAP = 180^\circ$ ⇒ $\angle APB + 72^\circ + 72^\circ = 180^\circ$ ⇒ $\angle APB = 36^\circ$ Hence, the correct answer is $36^\circ$.
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