Differentiation - Connected Rates of Change

Differentiation - Connected Rates of Change

TGM Original Questions

A curve has the equation \(y = x^3 + 4x^2 + kx + 3\), where \(k\) is a constant. Find, respectively, the range of values of \(k\) for which

  1. the curve is always increasing, and
  2. the curve has a stationary point at which \(\frac{\mathrm{d}^2 y}{\mathrm{d}x^2} = 0\).
  3. In the case where \(k = 6\), and the rate of increase of \(x\) is \(0.05\) unit/s when \(x = 1\), find the rate of change of \(y\) at this instant.

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Answer:(i) \(k\ge\dfrac{16}{3}\) (ii) \(k=\dfrac{16}{3}\) (iii) \(\dfrac{17}{20}\text{ unit s}^{-1}\)

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