Question triang

3.1 Ratios, Rates, and Proportional Relationships - Scale factors in geometry
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The figure presents two right triangles, A, B C and D E F. In triangle A, B C, side A, C is horizontal and side B C is drawn such that it is perpendicular to side A, C and point B, is above point C.  Angle A, is labeled 32 degrees and the right angle symbol is indied at angle C. In triangle D E F, side D F is horizontal, side E F is drawn such that it is perpendicular to side D F, and point E, is above point F. Angle D is labeled 58 degrees and the right angle symbol is indied at angle F.

Triangles ABC and DEF are shown above. Which of the following is equal to the ratio B C over A, B?

A.

D E over D F

B.

D F over D E

C.

D F over E F

D.

E F over D E

Triangles ABC and DEF are shown above. Which of the following is equal to the ratio

Hard-difficulty · SAT Math · Ratios, Rates, and Proportional Relationships — Scale factors in geometry. Read the question above, select your answer, and check the full explanation below to understand exactly why the correct choice works.

Answer explanation

Choice B is correct. In right triangle ABC, the measure of angle B must be 58° because the sum of the measure of angle A, which is 32°, and the measure of angle B is 90°. Angle D in the right triangle DEF has measure 58°. Hence, triangles ABC and DEF are similar (by angle-angle similarity). Since side B C is the side opposite to the angle with measure 32° and AB is the hypotenuse in right triangle ABC, the ratio the length of side B C over the length of side A, B is equal to the length of side D F over the length of side D E.

Alternate approach: The trigonometric ratios can be used to answer this question. In right triangle ABC, the ratio the length of side B C over the length of side A, B equals, the sine of 32 degrees. The angle E in triangle DEF has measure 32° because the measure of angle D, plus the measure of angle E, equals 90 degrees. In triangle DEF, the ratio the length of side D F over the length of side D E, equals the sine of 32 degrees. Therefore, the length of side D F over the length of side D E, equals, the length of side B C over the length of side A, B.

Choice A is incorrect because the length of side D E over the length of side D F is the reciprocal of the ratio the length of side B C over the length of side A, B. Choice C is incorrect because the length of side D F over the length of side D E, equals the length of side B C over the length of side A, C, not the length of side B C over the length of side A, B. Choice D is incorrect because the length of side E F over the length of side D E, equals, the length of side A, C over the length of side A, B, not the length of side B C over the length of side A, B.