2017 AMC 10B Problem 19

Below is the professionally curated solution for Problem 19 of the 2017 AMC 10B, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2017 AMC 10B solutions, or check the answer key.

All of the real AMC 8, AMC 10, AMC 12, and AIME problems in our complete solution collection are used with official legal permission of the Mathematical Association of America (MAA).

Concepts:area ratioequilateral triangletriangle area

Difficulty rating: 1860

19.

Let ABCABC be an equilateral triangle. Extend side AB\overline{AB} beyond BB to a point BB' so that BB=3AB.BB'=3 \cdot AB. Similarly, extend side BC\overline{BC} beyond CC to a point CC' so that CC=3BC,CC'=3 \cdot BC, and extend side CA\overline{CA} beyond AA to a point AA' so that AA=3CA.AA'=3 \cdot CA.

What is the ratio of the area of ABC\triangle A'B'C' to the area of ABC?\triangle ABC?

9:19:1

16:116:1

25:125:1

36:136:1

37:137:1

Solution:

We know that: [ABC]=[ABC]+[ABA]+[BCB]+[CAC].\begin{align*}[A'B'C'] =& [ABC]+ [A'B'A]\\ &+ [B'C'B] + [C'A'C] .\end{align*} The last three terms on the right hand side of the equation have the same area, so the area: [ABC]=[ABC]+3[ABA].[A'B'C']=[ABC]+3[A'B'A]. Therefore, to find the ratio in question, we need to find: [ABC]+3[ABA][ABC]\dfrac{[ABC]+3[A'B'A] }{[ABC]}=1+3[ABB][ABC].= 1+ 3\dfrac{[A'B'B]}{[ABC]}. Then, [ABB]=12AAAAsinAAB\begin{align*}[A'B'B] =& \frac12 A'A\cdot A'A \\&\cdot \sin A'AB' \end{align*} and [ABC]=12ABACsinBAC.[ABC] = \frac 12 AB\cdot AC \cdot \sin BAC. Since AAB\angle A'AB' and BAC\angle BAC are supplements, they have the same sine.

Therefore, [ABB][ABC]=AABAABAC.\dfrac{[A'B'B]}{[ABC]} = \dfrac{A'A \cdot B'A}{AB\cdot AC} . Then, AB=AB+BB=4AB,A'B = AB + BB' = 4AB, and AA=3AC.AA' = 3 AC . This makes [ABB][ABC]=43=12.\dfrac{[A'B'B]}{[ABC]} = 4\cdot 3 = 12. As such, the final ratio is 1+312=37.1+ 3\cdot 12 = 37.

Thus, the correct answer is E .

Problem 19 in Other Years