Bunuel wrote:
In the figure above, point O is the center of the semicircle, and PQ is parallel to OS.
What is the measure of ∠ROS?
(A) 34°
(B) 36°
(C) 54.5°
(D) 72°
(E) 73°
KAPLAN OFFICIAL SOLUTION:Analyze: What shapes do we see in this mess? A semicircle and three isosceles triangles (we know they’re isosceles because each of them has two sides that are radii of the semicircle).
Because they’re isosceles, what else can we label? We know that for all three triangles, the two “top” angles are equal. So, angle OPQ is 70°, angle OQR is x°, and ORS is x + 1°.
What else does the Q-stem tell us? That PQ is parallel to OS. That means we can find corresponding angles.
Where is a corresponding angle to angle OQP found? Angle QOS. It must also be 70°.
Why might that be of interest? Because PO and QO are both transversals.
Task: Good analysis. What are we being asked for? The measure of angle ROS.
And what do we know about triangles and their internal angles? They sum to 180°. so, < ROS + 2(x +1) = 180.
Approach strategically: All right. We know a lot about triangles QOR and ROS now. What will the sum of their combined angles be? 360. Each triangle has 180°.
Give us an equation that contains all the info we have. 360 = 70 + 4x + 2.
Solve that for x. 360 = 4x + 72 → 288 = 4x → 72 = x.
And how can we use that to calculate angle ROS? 180 = 2(72 + 1) + < ROS → 180 = 146 + < ROS → 34 = < ROS.
Answer (a) is correct.Confirm: That wasn’t so bad actually. Notice how we used triangles, circles, and even good ol’ lines and angles to get the solution. Now, what if you didn’t have time for those steps. Is there a guessing strategy available? The triangles are pretty close and the lines are parallel, so x° must be close to 70°. That would get us down to (a) or (B). Given that <QOR has angles of x° and < ROS has angles of (x + 1)°, it makes sense to go with (a).
By the way, what’s up with answers (D) and (E)? They’re traps in case we solved for x or (x + 1) instead of what we’re supposed to.
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