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Number theory Difficulty 5.7 AIME, harder Find the answer

Let the four-digit number w1=abcdw_{1}=\overline{a b c d} be a perfect square. By splitting it in the middle, we get two two-digit numbers x1=abx_{1}=\overline{a b} and y1=cd;w2=3x1y1+1y_{1}=\overline{c d} ; w_{2}=3 x_{1} y_{1}+1 is a perfect square. By splitting w2w_{2} in the middle, we get two two-digit numbers x2x_{2} and y2;w3=y_{2} ; w_{3}= 2x2y22 x_{2} y_{2} is a perfect square. By splitting w3w_{3} in the middle, we get two two-digit numbers x3x_{3} and y3;w4=x3y3+1y_{3} ; w_{4}=x_{3} y_{3}+1 is a perfect square, and 9 times w4w_{4} is the four-digit number w5,w_{5}, which is also a perfect square. Find the four-digit numbers wi(i=1,2,3,4,5)w_{i}(i=1,2,3,4,5).

A number or a short expression. Spacing and $ signs are ignored.

Solution

Solution: From the fact that 9 times w4w_{4} is the four-digit number w5w_{5}, and w5w_{5} is also a perfect square, we have
w4=332=1089,w5=9×332=992=9801. \begin{array}{l} w_{4}=33^{2}=1089, \\ w_{5}=9 \times 33^{2}=99^{2}=9801 . \end{array}

From w4=x3y3+1w_{4}=x_{3} y_{3}+1 being the perfect square 1089, we have
10891=3321=34×32=17×64=16×68. \begin{array}{l} 1089-1=33^{2}-1 \\ =34 \times 32=17 \times 64=16 \times 68 . \end{array}

Among the six four-digit numbers 1668, 6816, 6417, 1764, 3432, and 3234, only 1764=4221764=42^{2} is a perfect square, so w3=1764w_{3}=1764.
From w3=2x2y2w_{3}=2 x_{2} y_{2} being the perfect square 1764, we have
x2y2=882=21×42=14×63=18×49 x_{2} y_{2}=882=21 \times 42=14 \times 63=18 \times 49 \text {. }

Upon inspection, only 1849=4321849=43^{2} is a perfect square, so TJw2=1.849T J w_{2}=1.849.

From w2=3x1y1+1w_{2}=3 x_{1} y_{1}+1 being the perfect square 1849, we have 3x1y1=4321=44×423 x_{1} y_{1}=43^{2}-1=44 \times 42, thus
x1y1=44×14=22×28=11×56. x_{1} y_{1}=44 \times 14=22 \times 28=11 \times 56 .

Upon inspection, only 1156 and 1444 are perfect squares, so w1=1156w_{1}=1156 or 1444.
In summary, w1=1156=342w_{1}=1156=34^{2} or 1444=3821444=38^{2},
w2=1849=432,w3=1764=422,w4=1089=332,w5=9801=992. \begin{array}{l} w_{2}=1849=43^{2}, w_{3}=1764=42^{2}, \\ w_{4}=1089=33^{2}, w_{5}=9801=99^{2} . \end{array}
(Tian Yonghai, Suihua Educational Institute, Heilongjiang Province, 152054)

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.