Powers, powers, ugh!

Algebra Level 1

y + y 2 + y 3 + + y 60 + y 61 = 1 \large y + y^2 + y^3 + \cdots + y^{60} + y^{61} = -1

Given that y y is a real number, that is find the value of y y that satisfies the equation above.


The answer is -1.

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2 solutions

Hung Woei Neoh
Jun 8, 2016

y + y 2 + y 3 + + y 60 + y 61 = 1 y 61 + y 60 + y 59 + y 58 + + y 3 + y 2 + y + 1 = 0 y 60 ( y + 1 ) + y 58 ( y + 1 ) + + y 2 ( y + 1 ) + 1 ( y + 1 ) = 0 ( y + 1 ) ( y 60 + y 58 + + y 2 + 1 ) = 0 y + y^2 + y^3 + \ldots + y^{60} + y^{61} = -1\\ y^{61} + y^{60} + y^{59} + y^{58} + \ldots + y^3 + y^2 + y + 1 = 0\\ y^{60} (y+1) +y^{58}(y+1)+ \ldots + y^2(y+1) + 1(y+1) = 0\\ (y+1)(y^{60} + y^{58} + \ldots + y^2 + 1) = 0

The first factor:

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

The second factor:

y 60 + y 58 + + y 2 + 1 = 0 y^{60} + y^{58} + \ldots + y^2 + 1 = 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 60 0 , y 58 0 , , y 2 0 y^{60} \geq 0,\;y^{58} \geq 0, \ldots,\;y^2 \geq 0

Therefore,

y 60 + y 58 + + y 2 0 y 60 + y 58 + + y 2 + 1 1 y^{60} + y^{58} + \ldots + y^2 \geq 0\\ y^{60} + y^{58} + \ldots + y^2 + 1 \geq 1

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

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

Sonia Gupta
Jun 8, 2016

A number which doesn't change with its power, it is possible only when the number is 1. But according to the question the power on the number are even & odd number and the RHS is -1.(-1)² = 1 & (-1)³ = -1 . Like this all the evens & odds will cancel each other and the number which is left is (-1)/\61 = -1.

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