Maths Olympiad Prep

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Number theory Difficulty 5.6 AIME, harder Prove it Greece

Find all positive integers xx, yy, zz with zz odd, which satisfy the equation
2018x=100y+1918z 2018^x = 100^y + 1918^z

Solution

We write 2018=210092018 = 2 \cdot 1009, 100=2252100 = 2^2 \cdot 5^2 and 1918=271371918 = 2 \cdot 7 \cdot 137. Then, the equation has the form
2x1009x=22y52y+2z7z137z,(1) 2^x \cdot 1009^x = 2^{2y} \cdot 5^{2y} + 2^z \cdot 7^z \cdot 137^z, \quad (1)

Consider the following cases:

* If 2y<z2y < z, then the power of 22 that divides the right hand side is 22y2^{2y}. (2)
From (1) and (2) we get that x=2yx = 2y. The equation takes the form:
1918z=20182y100y=(2018y10y)(2018y+10y). 1918^z = 2018^{2y} - 100^{y} = (2018^y - 10^y)(2018^y + 10^y).
However, 20102018y10y2010 \mid 2018^y - 10^y, so 20101918z2010 \mid 1918^z, a contradiction.

* If z<2yz < 2y, then the highest power of 22 that divides the right hand side is
2z.(3)From (1) and (3) we get x=z. 2^z. \quad (3) \quad \text{From (1) and (3) we get } x = z.
Then the equation has the form:
2018z1918z=100y2z(1009z7z137z)=22y52y.(4) 2018^z - 1918^z = 100^y \Leftrightarrow 2^z (1009^z - 7^z \cdot 137^z) = 2^{2y} \cdot 5^{2y}. \quad (4)
Since zz is odd, the number 1009z7z137z1009^z - 7^z \cdot 137^z is even but it is not divisible by 44, so the highest power of 22 that divides the left hand side is z+1z+1, so 2y=z+12y = z+1, and the equation takes the form 2018z1918z=10z+12018^z - 1918^z = 10^{z+1}. Therefore, 10z+1=2018z1918z100z=102z10^{z+1} = 2018^z - 1918^z \ge 100^z = 10^{2z}, so 2zz+1z1z=12z \le z+1 \Leftrightarrow z \le 1 \Leftrightarrow z = 1.
It follows that the only solution is (x,y,z)=(1,1,1)(x, y, z) = (1, 1, 1).

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Source: MathNet, licensed CC-BY-4.0. Statement and solution reproduced as published; topic and difficulty added by this site.