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True or false: the vanishing of the first derivative at a point guarantees an extremum there.

  • A.True, unless the second derivative also vanishes
  • B.False, an inflexion with horizontal tangent also satisfying it
  • C.True, the condition being both necessary and sufficient
  • D.True, provided the function is continuous nearby

Answer: B

The condition is necessary for an interior extremum of a differentiable function but not sufficient. The function \( y = x^{3} \) has \( y' = 0 \) at the origin yet rises through it, the point being a horizontal inflexion. Deciding which case holds needs the second derivative, or the first non-vanishing one

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