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Algebra Level 4

( x 2 + 1 x ) + ( x x 2 + 1 ) = 2.9 \left(\dfrac{x^2+1}{x}\right)+\left(\dfrac{x}{x^2+1}\right)=2.9 Find the sum of all real values of x x satisfying the above equation.


The answer is 2.5.

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1 solution

Let y = x 2 + 1 x . y = \dfrac{x^{2} + 1}{x}. Then the given equation becomes

y + 1 y = 29 10 10 y 2 29 y + 10 = 0 ( 2 y 5 ) ( 5 y 2 ) = 0 , y + \dfrac{1}{y} = \dfrac{29}{10} \Longrightarrow 10y^{2} - 29y + 10 = 0 \Longrightarrow (2y - 5)(5y - 2) = 0,

and so either y = 5 2 y = \dfrac{5}{2} or y = 2 5 . y = \dfrac{2}{5}.

Now y = x 2 + 1 x = 5 2 2 x 2 5 x + 2 = 0 ( 2 x 1 ) ( x 2 ) = 0 , y = \dfrac{x^{2} + 1}{x} = \dfrac{5}{2} \Longrightarrow 2x^{2} - 5x + 2 = 0 \Longrightarrow (2x - 1)(x - 2) = 0,

and so either x = 2 x = 2 or x = 1 2 , x = \dfrac{1}{2}, both of which satisfy the original equation.

Next, y = x 2 + 1 x = 2 5 5 x 2 2 x + 5 = 0 , y = \dfrac{x^{2} + 1}{x} = \dfrac{2}{5} \Longrightarrow 5x^{2} - 2x + 5 = 0,

which has no real solutions as the discriminant b 2 4 a c = ( 2 ) 2 4 5 5 = 96 < 0. b^{2} - 4ac = (-2)^{2} - 4*5*5 = -96 \lt 0.

Thus the sum of all real solutions x x is 1 2 + 2 = 5 2 = 2.5 . \dfrac{1}{2} + 2 = \dfrac{5}{2} = \boxed{2.5}.

Did exactly the same way!

A Former Brilliant Member - 5 years, 7 months ago

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