CombinatoricsDifficulty 3.2AMC 10/12Find the answer
Let n be a positive integer. If the equation 2x+2y+z=n has 28 solutions in positive integers x, y, and z, then n must be either (A)14 or 15(B)15 or 16(C)16 or 17(D)17 or 18(E)18 or 19
Multiple choice: answer with the letter of the option you want.
Solution
This is equivalent to finding the powers of k with coefficient 28 in the expansion of (k2+k4+k6+k8+...)2(k+k2+k3+k4+...). But this is (1−k2k2)2⋅1−kk=k5⋅(1−k1)3⋅(1+k1)2=k5⋅(1+k)⋅(1−k21)3 =k5(1+k)((22)+(23)k2+(24)k4+(25)k6+...), the last part having general term (2j)k2j−4 from which it is easy to see that, since (28)=28, the last part contains the term 28k12 and the whole result 28k17+28k18. So the answer is (D).
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Source: NuminaMath-1.5,
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