Powers, powers, ugh! (Part 2)

Algebra Level 1

y 2 + y 3 + + y 60 + + y 99 = 0 \large y^2 + y^3 + \cdots + y^{60} + \cdots + y^{99} = 0

Find a non-zero real number y y that satisfies the equation above.


The answer is -1.

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1 solution

Hung Woei Neoh
Jun 10, 2016

y 2 + y 3 + + y 60 + + y 99 = 0 y 2 ( 1 + y + y 2 + y 3 + + y 94 + y 95 + y 96 + y 97 ) = 0 y 2 ( 1 ( y + 1 ) + y 2 ( y + 1 ) + + y 94 ( y + 1 ) + y 96 ( y + 1 ) ) = 0 y 2 ( y + 1 ) ( 1 + y 2 + + y 94 + y 96 ) = 0 y^2+y^3 + \ldots+y^{60} + \ldots+y^{99} = 0\\ y^2(1+y+y^2+y^3+\ldots+y^{94}+y^{95}+y^{96}+y^{97}) = 0\\ y^2\left(1(y+1) + y^2(y+1) + \ldots +y^{94}(y+1) + y^{96}(y+1)\right) = 0\\ y^2(y+1)(1+y^2+\ldots+y^{94} + y^{96}) = 0

The first factor:

y 2 = 0 y = 0 y^2 = 0 \implies y = 0

This is not non-zero, so we ignore it.

The second factor:

y + 1 = 0 y = 1 y+1=0 \implies y=-1

The third factor

1 + y 2 + + y 94 + y 96 = 0 1+y^2+\ldots+y^{94}+y^{96} = 0

Notice that for even positive integers of n n , y n 0 y^n \geq 0 for all real values of y y

This implies that y 2 0 , , y 94 0 , y 96 0 y^{2} \geq 0, \ldots,\;y^{94} \geq 0,\;y^{96} \geq 0

Therefore,

y 2 + + y 94 + y 96 0 1 + y 2 + + y 94 + y 96 1 y^2+ \ldots +y^{94} + y^{96} \geq 0\\ 1+y^2+ \ldots+ y^{94} + y^{96} \geq 1

From this, we can conclude that the third factor has no real roots.

Therefore, the only non-zero real solution is y = 1 y=\boxed{-1}

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