Tricky equation

Algebra Level 3

How many real solution does the equation below have? x 4 + 2 x 3 + 3 x 2 + 2 x + 1 = 0 x^4+2x^3+3x^2+2x+1=0

1 3 2 4 0

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1 solution

Solution 1:

x 4 + 2 x 3 + 3 x 2 + 2 x + 1 = ( x 2 + x + 1 ) 2 = 0 x^4+2x^3+3x^2+2x+1=(x^2+x+1)^2=0

x 2 + x + 1 = 0 \Rightarrow x^2+x+1=0

( x + 1 ) 2 x = 0 \Rightarrow (x+1)^2-x=0

( x + 1 ) 2 = x \Rightarrow (x+1)^2=x

Since ( x + 1 ) 2 0 (x+1)^2\geq0 , x x has to be a non-negativ number, but it contradict that x 2 + x + 1 = 0 x^2+x+1=0 . So the equation hasn't got any solution.

Solution 2:

( x 4 + 2 x 3 + x 2 ) + x 2 + ( x 2 + 2 x + 1 ) = 0 (x^4+2x^3+x^2)+x^2+(x^2+2x+1)=0

x 2 ( x + 1 ) 2 + x 2 + ( x + 1 ) 2 = 0 \Rightarrow x^2(x+1)^2+x^2+(x+1)^2=0

On the left side there are only perfect squares, so the left side is non-negativ. If the sum of three non-negativ numbers is 0 0 , then all of them is 0. But x 2 x^2 and ( x + 1 ) 2 (x+1)^2 can't be 0 0 at the same time, so the equation hasn't got any solution.

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