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Number theory Difficulty 5.9 AIME, harder Find the answer

Find all nonnegative integer solutions (x,y,z,w)(x,y,z,w) of the equation2x3y5z7w=1.2^x\cdot3^y-5^z\cdot7^w=1.

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

Solution

We are tasked with finding all nonnegative integer solutions (x,y,z,w)(x, y, z, w) to the equation:
2x3y5z7w=1. 2^x \cdot 3^y - 5^z \cdot 7^w = 1.

First, we note that x1x \geq 1 because if x=0x = 0, the left-hand side would be a fraction, which cannot equal 1.

### Case 1: w=0w = 0
The equation simplifies to:
2x3y=1+5z. 2^x \cdot 3^y = 1 + 5^z.

- **Subcase 1.1: z=0z = 0**
2x3y=1. 2^x \cdot 3^y = 1.
This implies x=0x = 0 and y=0y = 0, but x1x \geq 1, so this is not possible.

- **Subcase 1.2: z=1z = 1**
2x3y=6. 2^x \cdot 3^y = 6.
The possible solutions are x=1x = 1 and y=1y = 1.

- **Subcase 1.3: z2z \geq 2**
Taking modulo 4, we get:
2x3y1(mod4). 2^x \cdot 3^y \equiv 1 \pmod{4}.
Since 2x0(mod4)2^x \equiv 0 \pmod{4} for x2x \geq 2, this leads to a contradiction.

### Case 2: w=1w = 1
The equation simplifies to:
2x3y=1+75z. 2^x \cdot 3^y = 1 + 7 \cdot 5^z.

- **Subcase 2.1: z=0z = 0**
2x3y=8. 2^x \cdot 3^y = 8.
The possible solutions are x=3x = 3 and y=0y = 0.

- **Subcase 2.2: z=1z = 1**
2x3y=36. 2^x \cdot 3^y = 36.
The possible solutions are x=2x = 2 and y=2y = 2.

- **Subcase 2.3: z2z \geq 2**
Taking modulo 8, we get:
2x1(mod8). 2^x \equiv 1 \pmod{8}.
This implies x=0x = 0, which is not possible.

### Case 3: w2w \geq 2
The equation becomes:
2x3y=1+5z7w. 2^x \cdot 3^y = 1 + 5^z \cdot 7^w.

- **Subcase 3.1: y=0y = 0**
2x=1+5z7w. 2^x = 1 + 5^z \cdot 7^w.
Taking modulo 4, we get a contradiction for x2x \geq 2.

- **Subcase 3.2: y1y \geq 1**
Taking modulo 9, we get a contradiction for z2z \geq 2.

Thus, we have exhausted all possible cases and find the solutions to be:
(1,1,1,0),(2,2,1,1),(1,0,0,0),(3,0,0,1). \boxed{(1, 1, 1, 0), (2, 2, 1, 1), (1, 0, 0, 0), (3, 0, 0, 1)}.

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