Find the number of real roots of the polynomial 3 x 5 − 2 5 x 3 + 6 0 x .
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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.
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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.
how did u use Descartes’ rule of signs here?
graph shows that it has only one real solution at x = 0 , and we can also prove it like this.
℘ ( x ) = x ( 3 x 4 − 2 5 x 2 + 6 0 )
This gives the trivial root at x = 0
now let x 2 = K
new equation comes as 3 k 2 − 2 5 k + 6 0
since its determinant is negetive so it will not provide any real solution.
Hence, the number of real roots of , ℘ ( x ) , = 1
℘ ( x ) = x ( 3 x 4 − 2 5 x 2 + 6 0 )
Let x 2 = y ⇒ x ( 3 y 2 − 2 5 y + 6 0 )
Solving 3 y 2 − 2 5 y + 6 0 we get that y = 6 2 5 ± − 9 5 ∴ x = 0 in the only real root.
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
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At the get-go, factor out the x from the equation. Then,
℘ ( x ) = x ( 3 x 4 − 2 5 x 2 + 6 0 )
This gives the trivial root of x = 0 .
Set t = x 2 . Then, the bi-quadratic reduces to :
3 t 2 − 2 5 t + 6 0
By eyeballing it, one can determine that the 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 ) , N = 1