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Number theory Difficulty 4.7 AIME Find the answer

Solve in positive integers the equation 1005x+2011y=1006z1005^{x}+2011^{y}=1006^{z}.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

We have 1006z>2011y>20111006^{z}>2011^{y}>2011, hence z2z \geq 2. Then 1005x+2011y0(mod4)1005^{x}+2011^{y} \equiv 0(\bmod 4).

But 1005x1(mod4)1005^{x} \equiv 1(\bmod 4), so 2011y1(mod4)y2011^{y} \equiv-1(\bmod 4) \Rightarrow y is odd, i.e. 2011y1(mod1006)2011^{y} \equiv-1(\bmod 1006).

Since 1005x+2011y0(mod1006)1005^{x}+2011^{y} \equiv 0(\bmod 1006), we get 1005x1(mod1006)x1005^{x} \equiv 1(\bmod 1006) \Rightarrow x is even.

Now 1005x1(mod8)1005^{x} \equiv 1(\bmod 8) and 2011y3(mod8)2011^{y} \equiv 3(\bmod 8), hence 1006z4(mod8)z=21006^{z} \equiv 4(\bmod 8) \Rightarrow z=2.

It follows that y<2y=1y<2 \Rightarrow y=1 and x=2x=2. The solution is (x,y,z)=(2,1,2)(x, y, z)=(2,1,2).

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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.