Soal Geometri: Similarity (problem 1)

Problem 1:

In the triangle ABC shown below, A’C’ is parallel to AC. Find the length y of BC’ and the length x of A’A.

similar_tri_problems_1

Solution to Problem 1:
BA is a transversal that intersects the two parallel lines A’C’ and AC, hence the corresponding angles BA’C’ and BAC are congruent. BC is also a transversal to the two parallel lines A’C’ and AC and therefore angles BC’A’ and BCA are congruent. These two triangles have two congruent angles are therefore similar and the lengths of their sides are proportional. Let us separate the two triangles as shown below.

similar_tri_problems_2

We now use the proportionality of the lengths of the side to write equations that help in solving for x and y.
(30 + x) / 30 = 22 / 14 = (y + 15) / y
An equation in x may be written as follows.
(30 + x) / 30 = 22 / 14
Solve the above for x.

420 + 14 x = 660

x = 17.1 (rounded to one decimal place).
An equation in y may be written as follows.

22 / 14 = (y + 15) / y
Solve the above for y to obtain.

y = 26.25

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