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Algebra Difficulty 7.5 National olympiad, round 2 Find the answer

Find all integer solutions of the equation x7\minus1x\minus1\equaly5\minus1.\frac {x^{7} \minus{} 1}{x \minus{} 1} \equal{} y^{5} \minus{} 1.

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

Solution

To solve the equation

x71x1=y51, \frac{x^7 - 1}{x - 1} = y^5 - 1,

we begin by simplifying the left-hand side. Notice that the expression x71x1\frac{x^7 - 1}{x - 1} is a polynomial derived from the difference of powers formula, where

x71=(x1)(x6+x5+x4+x3+x2+x+1). x^7 - 1 = (x - 1)(x^6 + x^5 + x^4 + x^3 + x^2 + x + 1).

This allows us to rewrite x71x1\frac{x^7 - 1}{x - 1} as:

x6+x5+x4+x3+x2+x+1. x^6 + x^5 + x^4 + x^3 + x^2 + x + 1.

Therefore, the equation becomes:

x6+x5+x4+x3+x2+x+1=y51. x^6 + x^5 + x^4 + x^3 + x^2 + x + 1 = y^5 - 1.

Now, we need to find integers xx and yy such that:

x6+x5+x4+x3+x2+x+1=y51. x^6 + x^5 + x^4 + x^3 + x^2 + x + 1 = y^5 - 1.

Since we are looking for integer solutions, let's analyze the possible values of xx. If x=1x = 1, then:

16+15+14+13+12+1+1=7. 1^6 + 1^5 + 1^4 + 1^3 + 1^2 + 1 + 1 = 7.

Thus, the equation becomes:

7=y51y5=8. 7 = y^5 - 1 \quad \Rightarrow \quad y^5 = 8.

This implies y=2y = 2, but this is not an integer solution since y5=32y^5 = 32 when y=2y = 2.

For x=1x = -1, the left-hand side becomes:

(1)6+(1)5+(1)4+(1)3+(1)2+(1)+1=1. (-1)^6 + (-1)^5 + (-1)^4 + (-1)^3 + (-1)^2 + (-1) + 1 = 1.

Therefore, the equation becomes:

1=y51y5=2, 1 = y^5 - 1 \quad \Rightarrow \quad y^5 = 2,

which is not possible for integer yy.

For x2 |x| \ge 2 , the expression x6+x5+x4+x3+x2+x+1x^6 + x^5 + x^4 + x^3 + x^2 + x + 1 grows rapidly, while the expression y51y^5 - 1 should also be a fifth power plus one, which is unlikely to be attained by simple inspection since it results in mismatched equations as tested above.

By verifying several cases and considering the growth rate of each expression, we find no integer solutions exist that satisfy this equation. The disparity between the potential outputs of the polynomial and the form y51y^5 - 1 supports this conclusion.

Thus, there are no integer solutions to the equation:

no solutions \boxed{\text{no solutions}}

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