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Algebra Level 2

Find the number of real roots of the polynomial 3 x 5 25 x 3 + 60 x 3x^{5} -25x^{3} +60x .


The answer is 1.

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5 solutions

This is the non-Calculus solution :

At the get-go, factor out the x x from the equation. Then,

( x ) = x ( 3 x 4 25 x 2 + 60 ) \displaystyle \large {\wp(x)} = x(3x^{4} -25x^{2} +60)

This gives the trivial root of x = 0 x = 0 .

Set t = x 2 \displaystyle t = x^2 . Then, the bi-quadratic reduces to :

3 t 2 25 t + 60 \displaystyle 3t^{2} -25t +60

By eyeballing it, one can determine that the D \large D for this quadratic is less than 0, consequently proving that the said quadratic( and hence bi-quadratic) has no real solutions.

Hence, the number of real roots of ( x ) \displaystyle \large {\wp(x)} , N = 1 \displaystyle \large N = \boxed{1}

I solved with the discriminant as well. This question is overrated in my opinion, but it seems tedious at first sight. Descartes' rule of signs is also helpful here.

Caleb Townsend - 6 years, 3 months ago

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Yeah...Even I tried by doing it with the help of the Intermediate Value Theorem, but I noticed this and thought to myself "Oh god! This is so simple! Why do I need to come to the answer in a longer way?!", and hence the solution.

B.S.Bharath Sai Guhan - 6 years, 3 months ago

how did u use Descartes’ rule of signs here?

Vandit Kumar - 3 years, 3 months ago
Aniket Verma
Mar 7, 2015

graph shows that it has only one real solution at x = 0 x= 0 , and we can also prove it like this.

( x ) = x ( 3 x 4 25 x 2 + 60 ) \displaystyle \large {\wp(x)} = x(3x^4 - 25x^2 + 60)

This gives the trivial root at x = 0 x=0

now let x 2 = K x^2 = K

new equation comes as 3 k 2 25 k + 60 3k^2 -25k + 60

since its determinant is negetive so it will not provide any real solution.

Hence, the number of real roots of , ( x ) \displaystyle \large {\wp(x)} , = 1 = 1

Paola Ramírez
Mar 5, 2015

( x ) = x ( 3 x 4 25 x 2 + 60 ) \wp(x)=x(3x^4-25x^2+60)

Let x 2 = y x ( 3 y 2 25 y + 60 ) x^2=y \Rightarrow x(3y^2-25y+60)

Solving 3 y 2 25 y + 60 3y^2-25y+60 we get that y = 25 ± 95 6 x = 0 y=\frac{25 \pm \sqrt{-95}}{6} \therefore \boxed{x=0} in the only real root.

Daniel Yang
Mar 20, 2015

Real roots come in pairs. Th only possible values are 5,3,1. Type them in and you get the right answer!

You have to love this answer. Ed Gray

Edwin Gray - 3 years, 2 months ago

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