A function f is defined by f : x ↦ (2x − 3)3 − 8, for 2 ≤ x ≤ 4.
Find an expression, in terms of x, for f−1(x) and find the domain of f−1.
Solution
\(Let y = (2x − 3)3 − 8.\)
To find the inverse, solve for x in terms of y:
\((2x − 3)3 = y + 8\)
\(2x − 3 = \sqrt[3]{y + 8}\)
\(2x = \sqrt[3]{y + 8} + 3\)
\(x = \frac{\sqrt[3]{y + 8} + 3}{2}\)
\(Thus, f−1(x) = \frac{\sqrt[3]{x + 8} + 3}{2}.\)
The range of f is the domain of f−1. Calculate the range of f for 2 ≤ x ≤ 4:
\(For x = 2, f(2) = (2(2) − 3)3 − 8 = −7.\)
\(For x = 4, f(4) = (2(4) − 3)3 − 8 = 117.\)
Therefore, the domain of f−1 is −7 ≤ x ≤ 117.
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