We need to find the measure of angle $R$, denoted as $m\angle R$. We are given that the segments that appear to be tangent are tangent. We are also given that the intercepted arc $PQ$ is $40^\circ$ and the angle formed by the two tangents outside the circle at point $V$ is $120^\circ$.

GeometryCircle GeometryTangentsAngles in a CircleArcs
2025/5/9

1. Problem Description

We need to find the measure of angle RR, denoted as mRm\angle R. We are given that the segments that appear to be tangent are tangent. We are also given that the intercepted arc PQPQ is 4040^\circ and the angle formed by the two tangents outside the circle at point VV is 120120^\circ.

2. Solution Steps

Let mV=120m\angle V = 120^\circ and mPQ=40m\stackrel{\frown}{PQ} = 40^\circ.
We have the formula for the angle formed by two tangents drawn to a circle from an exterior point:
mV=12(mPTQmPQ)m\angle V = \frac{1}{2} (m\stackrel{\frown}{PTQ} - m\stackrel{\frown}{PQ})
where PTQPTQ is the major arc and PQPQ is the minor arc. The entire circle is 360360^\circ, so mPTQ=360mPQ=36040=320m\stackrel{\frown}{PTQ} = 360^\circ - m\stackrel{\frown}{PQ} = 360^\circ - 40^\circ = 320^\circ.
Then
120=12(32040)=12(280)=140120^\circ = \frac{1}{2} (320^\circ - 40^\circ) = \frac{1}{2} (280^\circ) = 140^\circ.
This is a contradiction! Therefore, we must consider the tangents to intersect at R.
Let xx be the mTQm\stackrel{\frown}{TQ}. Then mR=12(mPQmTQ)m\angle R = \frac{1}{2}(m\stackrel{\frown}{PQ} - m\stackrel{\frown}{TQ})
40=12(mPQ)40^\circ = \frac{1}{2} (m\stackrel{\frown}{PQ})
so mPQ=240=80m\stackrel{\frown}{PQ} = 2 * 40^\circ = 80^\circ.
Also mPT+mTQ+mPQ=360m\stackrel{\frown}{PT} + m\stackrel{\frown}{TQ} + m\stackrel{\frown}{PQ} = 360^\circ.
We also know that
mV=12(mPTmTQ)m\angle V = \frac{1}{2} (m\stackrel{\frown}{PT} - m\stackrel{\frown}{TQ})
120=12(mPTmTQ)120^\circ = \frac{1}{2} (m\stackrel{\frown}{PT} - m\stackrel{\frown}{TQ}).
Thus, 240=mPTmTQ240^\circ = m\stackrel{\frown}{PT} - m\stackrel{\frown}{TQ}.
We have mPT+mTQ+40=360m\stackrel{\frown}{PT} + m\stackrel{\frown}{TQ} + 40^\circ = 360^\circ.
mPT+mTQ=320m\stackrel{\frown}{PT} + m\stackrel{\frown}{TQ} = 320^\circ.
So, mPT=320mTQm\stackrel{\frown}{PT} = 320^\circ - m\stackrel{\frown}{TQ}.
240=320mTQmTQ240^\circ = 320^\circ - m\stackrel{\frown}{TQ} - m\stackrel{\frown}{TQ}.
2mTQ=320240=802m\stackrel{\frown}{TQ} = 320^\circ - 240^\circ = 80^\circ.
mTQ=40m\stackrel{\frown}{TQ} = 40^\circ.
Therefore mPT=32040=280m\stackrel{\frown}{PT} = 320^\circ - 40^\circ = 280^\circ.
The question asks for angle R.
mR=12(mPTmPQ)=12(28080)=12(200)=100m\angle R = \frac{1}{2} (m\stackrel{\frown}{PT} - m\stackrel{\frown}{PQ}) = \frac{1}{2} (280^\circ - 80^\circ) = \frac{1}{2} (200^\circ) = 100^\circ.

3. Final Answer

100

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