A function f is defined for x ∈ ℝ and is such that f'(x) = 2x − 6. The range of the function is given by f(x) ≥ −4.
- State the value of x for which f(x) has a stationary value.
- Find an expression for f(x) in terms of x.
Solution
(i) To find the stationary point, set the derivative equal to zero:
\(f'(x) = 2x - 6 = 0\)
Solving for \(x\), we get:
\(2x = 6\)
\(x = 3\)
Thus, the value of \(x\) for which \(f(x)\) has a stationary value is 3.
(ii) To find \(f(x)\), integrate \(f'(x)\):
\(f(x) = \int (2x - 6) \, dx\)
\(f(x) = x^2 - 6x + c\)
Given \(f(x) \geq -4\), substitute \(x = 3\) to find \(c\):
\(f(3) = 3^2 - 6(3) + c = -4\)
\(9 - 18 + c = -4\)
\(c = 5\)
Thus, \(f(x) = x^2 - 6x + 5\).
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