Question

Meg constructed triangle POQ and then used a compass and straightedge to accurately construct line segment OS, as shown in the figure below:
Which could be the measures of angle POS and angle POQ?

Measure of angle POS is 25 degrees, measure of angle POQ is 40 degrees
Measure of angle POS is 25 degrees, measure of angle POQ is 30 degrees
Measure of angle POS is 20 degrees, measure of angle POQ is 40 degrees
Measure of angle POS is 20 degrees, measure of angle POQ is 30 degrees

Answer 1

In order to understand this question, one needs to understand the construction first. After creating POQ, Meg picks a radius r and makes a circle around O. There are 2 points where the circle meets the triangle. Then, she meets another radius and she makes a circle of radius s around each of the points on the triangle. The two circles cross at one point, as can be seen in the figure. Let's name the first two points A and B and let's name the last point S. We have that for the triangles OAS, OBS we have OA=OB=r, BS=AS=s and that OS is common to both. Thus by the S-S-S triangle equality theorem, the triangles are equal. Hence for the angles AOS=BOS, namely POS=QOS. Since POQ=POS+QOS, we have that POS has to be half of POQ. The only consistent choice is the third one, POS=20 degrees and POQ=40 degrees.

Answer 2

The correct answer is:

Measure of angle POS is 20 degrees, measure of angle POQ is 40 degrees

Explanation:

OS is an angle bisector of ∠POQ.  This means it splits ∠POQ into two congruent pieces.  

This tells us that ∠POS+∠SOQ = ∠POS+∠POS = ∠POQ.

If m∠POS = 20, this means that m∠POQ = 20+20 = 40°.