Roots

Calculus Level 3

How many positive roots does the equation x 7 + 14 x 5 + 17 x 3 + 15 x 22 = 0 x^7 + 14 x^5 + 17 x^3 + 15x - 22 = 0 have?

6 0 7 4 5 2 1 3

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1 solution

Let f ( x ) = x 7 + 14 x 5 + 17 x 3 + 15 x 22 f(x) = x^7 + 14 x^5 + 17 x^3 + 15x - 22

f ( 0 ) = 22 < 0 f(0) = -22 < 0

f ( x ) = 7 x 6 + 70 x 4 + 51 x 2 + 15 > 0 f'(x) = 7x^6 + 70 x^4 + 51x^2 + 15 > 0

For x > 0 , f ( x ) x > 0, f(x) is monotonic increasing for x R . x \in R.

\therefore Number of roots = 1, and it is positive since f ( 0 ) < 0. f(0) < 0.

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