2022 AMC 10A Problem 20

Attempt Problem 20 of the 2022 AMC 10A below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2022 AMC 10A solutions, or check the answer key.

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20.

A four-term sequence is formed by adding each term of a four-term arithmetic sequence of positive integers to the corresponding term of a four-term geometric sequence of positive integers. The first three terms of the resulting four-term sequence are 57,57, 60,60, and 91.91. What is the fourth term of this sequence?

190190

194194

198198

202202

206206

Answer: E
Concepts:arithmetic sequencegeometric sequenceDiophantine Equation
Difficulty rating: 2230
Solution:

Let the arithmetic sequence be a,a+d,a+2d,a+3d a, a + d, a + 2d, a + 3d and the geometric sequence be b,br,br2,br3. b, br, br^2, br^3.

Then a+b=57,(1) a + b = 57 \tag*{(1)}, a+d+br=60,(2) a + d + br = 60 \tag*{(2)}, and a+2d+br2=91.(3) a + 2d + br^2 = 91 \tag*{(3)}.

Subtracting (1)(1) from (2)(2) and (2)(2) from (3),(3), we get d+b(r1)=3 d + b(r - 1) = 3 and d+br(r1)=31. d + br(r - 1) = 31.

Subtracting these, we get b(r1)2=28. b(r - 1)^2 = 28.

Let t=b(r1)=brb,t=b(r-1)=br-b, which is an integer. Then t2=28b,t^2=28b, so t=14ut=14u and b=7u2b=7u^2 for some nonzero integer u.u. Because a=57b>0,a=57-b>0, we must have u{2,1,1,2}.u\in\{-2,-1,1,2\}.

Now r=1+t/b=1+2/u.r=1+t/b=1+2/u. The cases u=1u=-1 and u=2u=-2 give r=1r=-1 and r=0,r=0, respectively, so they cannot produce a positive geometric sequence. If u=2,u=2, then b=28,r=2,a=29,b=28, r=2, a=29, and d=3t=25,d=3-t=-25, making the third arithmetic term negative.

Therefore u=1,u=1, so b=7,r=3,a=50,b = 7, r = 3, a = 50, and d=11.d = -11. The arithmetic sequence is 50,39,28,17, 50,39,28,17, and the geometric sequence is 7,21,63,189. 7,21,63,189.

The desired answer is 17+189=206.17 + 189 = 206.

Thus, E is the correct answer.

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