Answer:
- (b) ΔRST ~ ΔRUS ~ ΔSUT
- (a) a = 400/21; b = 580/21
Explanation:
1.
Similarity statements are written with corresponding vertices in the same order. So, there are two tasks involved: (1) identifying corresponding vertices; (2) making sure they are written in the same order in the similarity statement.
Here, we could go to the trouble to identify all pairs of corresponding vertices. Then we could write our own similarity statements and see which answer choice corresponds. However, it is sufficient to do just enough work to allow elimination of bad answer choices.
Sometimes in figures like this, it is difficult to tell the short side of the right triangle from the long side. (This figure is pretty helpful.) However, it is always possible to identify the hypotenuse of each triangle. They are ...
RT, RS, ST
What this means is that these letters should appear in the same spots in each part of the similarity statement.
(a) RT are in the 1st and 3rd spots of the first triangle name. In the second triangle name, those spots are occupied by SU--not a hypotenuse. (rejected)
(b) The 1st and 3rd spots of each statement are occupied by RT, RS, ST, so this is a viable candidate.
(c) The 1st and 3rd spots of the first triangle name are occupied by ST, but those spots of the second triangle name are SU--not a hypotenuse. (rejected)
(d) The 1st and 2nd spots of the first triangle name are occupied by RT, and those spots in the second triangle name are RS. However, those spots in the third triangle name are TU--not a hypotenuse (rejected).
The only viable choice for similarity statement is the second choice:
ΔRST ~ ΔRUS ~ ΔSUT
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2.
We can find segment 'a' by writing a proportion between sides of the largest and medium-size triangles:
hypotenuse/long side = (21 +a)/29 = 29/21
21(21 +a) = 29² . . . . . multiply by 29·21
441 +21a = 841 . . . . simplify
21a = 400 . . . . . . . subtract 441
a = 400/21 . . . . . . . . . . . divide by 21. (eliminates choices C and D)
At this point, we can solve for b in similar fashion. However, it is sufficient to use "triangle sense" to eliminate the remaining bad choice. We know the hypotenuse of any right triangle is the longest side. Here, that means ...
b > 20 . . . . . (eliminates choice B)
The only viable answer choice is A:
a = 400/21, b = 580/21
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Additional comments
For multiple-choice questions, you can arrive at a solution just as effectively by eliminating bad choices as you can by working the problem in detail to develop the correct choice. Here, we try to use "common sense" to simplify the work as much as possible. This saves time, which is what you want when you're working a timed assignment.
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This geometry has several interesting features. If you start at the right angle and write side lengths in order clockwise, the two "inside triangles" have the order {longest side, hypotenuse, shortest side}. The overall outside triangle has the order {shortest side, hypotenuse, longest side}. That is, the order of the outside triangle vertices is counterclockwise for the same order of corresponding sides. Realizing/remembering this can immensely simplify the process of writing similarity statements for these triangles.
Another feature is one we demonstrated in the solution for 'a' in the second problem: the length of a segment touching the longest hypotenuse is the geometric mean of the lengths of the hypotenuse segments it touches. In the case in point: 29 = √((21)(21 +a)). We could also write the relationship: 20 = √((21)(a)), perhaps getting there more directly: a=400/21. And finally, b = √((a(a+21)). These geometric mean relationships all derive from the proportions you can write using triangle similarity.
(The geometric mean of two numbers is the square root of their product. For n numbers, it is the n-th root of their product.)