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Question: Answered & Verified by Expert
Let \( f(x)=a x^{3}+5 x^{2}+c x+1 \) be a polynomial function. If \( f(x) \) has extreme at \( x=\alpha \) and \( \beta \) such that \( \alpha \beta < 0 \) and \( f(\alpha) f(\beta) < 0 \), then the equation \( f(x)=0 \) has
MathematicsApplication of DerivativesJEE Main
Options:
  • A three distinct real roots.
  • B one positive root, if \( f(\alpha) < 0 \) and \( f(\beta)>0 \).
  • C one negative root, if \( f(\alpha)>0 \) and \( f(\beta) < 0 \).
  • D All of the above
Solution:
2111 Upvotes Verified Answer
The correct answer is: All of the above

Given,

αβ<0

α and β are of opposite signs.

Let α<0 and β>0.

It is given that fx has extremum at x=α, β.

Therefore, α and β are two distinct real roots of f'x=0.

But we know that between two distinct real roots of a polynomial, there is at least one real root of its derivative.

Therefore, fx has three distinct real roots λ, μ and v (say) such that λ<α<μ<β<v.

Thus, first option is correct.

If fx=0 has exactly one positive root, then it is evident from the figure that v>0 and λ, μ<0.

Therefore, α<μ<0

fαf0<0 [μ lies between α and 0]

fα<0 f0=1>0

 fβ>0 fαfβ<0

Thus, second option is also correct.

If fx=0 has exactly one negative real root, then from the figure, we have λ<0 and μ, v>0.

 0<μ<β

f0fβ<0 [μ lies between 0 and β]

fβ<0 f0=1>0

Thus, third option is also correct.

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