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Algebra Difficulty 5.8 AIME, harder Find the answer

Find all the integers x,yx, y and zz greater than or equal to 00 such that 2x+97y=z32^x + 9 \cdot 7^y = z^3

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

Solution

We are tasked with finding all non-negative integer solutions (x,y,z)(x, y, z) to the equation:

2x+97y=z3 2^x + 9 \cdot 7^y = z^3

Given that our solution must satisfy integer constraints and each variable is greater than or equal to zero, we will systematically explore potential solutions.

### Step 1: Analyze small values of zz
Let's begin by examining small values for zz since the cubic function grows rapidly and only a finite number of candidates will exist.

#### Sub-step: Calculate for z=0,1,2,3,4z = 0, 1, 2, 3, 4
z=0:2x+97y=03=0 z = 0: \quad 2^x + 9 \cdot 7^y = 0^3 = 0

This forces 2x=02^x = 0 and 97y=09 \cdot 7^y = 0, which is impossible for x,y0x, y \geq 0.

z=1:2x+97y=13=1 z = 1: \quad 2^x + 9 \cdot 7^y = 1^3 = 1

This can only be satisfied by 2x=12^x = 1 and 97y=09 \cdot 7^y = 0, which is impossible for integer y0y \geq 0.

z=2:2x+97y=23=8 z = 2: \quad 2^x + 9 \cdot 7^y = 2^3 = 8

Testing for x=0,1,2,3x = 0, 1, 2, 3:
- x=3x = 3: 23=82^3 = 8, so 97y=09 \cdot 7^y = 0, yy cannot be satisfied for y0y \geq 0.

z=3:2x+97y=33=27 z = 3: \quad 2^x + 9 \cdot 7^y = 3^3 = 27

Testing for small values of xx and yy:
- x=0x = 0: 20+970=1+9=102^0 + 9 \cdot 7^0 = 1 + 9 = 10, not 2727.
- x=1x = 1: 21+970=2+9=112^1 + 9 \cdot 7^0 = 2 + 9 = 11, not 2727.
- x=2x = 2: 22+970=4+9=132^2 + 9 \cdot 7^0 = 4 + 9 = 13, not 2727.

Trying with different values of yy, we find that no combination of (x,y)(x, y) results in 272x=97y27 - 2^x = 9 \cdot 7^y.

z=4:2x+97y=43=64 z = 4: \quad 2^x + 9 \cdot 7^y = 4^3 = 64

For x=0x = 0, solve 1+97y=641 + 9 \cdot 7^y = 64:
- 97y=639 \cdot 7^y = 63 which yields 7y=77^y = 7 and hence y=1y = 1.

The solution (x,y,z)=(0,1,4)(x, y, z) = (0, 1, 4) satisfies 20+971=1+63=64=432^0 + 9 \cdot 7^1 = 1 + 63 = 64 = 4^3.

### Conclusion
Therefore, the only solution with x,y,z0x, y, z \geq 0 satisfying the equation is:

(0,1,4) \boxed{(0, 1, 4)}

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