June 2015 p12 q1
1242
The function \(f\) is such that \(f'(x) = 5 - 2x^2\) and \((3, 5)\) is a point on the curve \(y = f(x)\). Find \(f(x)\).
Solution
To find \(f(x)\), we need to integrate \(f'(x) = 5 - 2x^2\).
\(\int (5 - 2x^2) \, dx = \int 5 \, dx - \int 2x^2 \, dx\)
\(= 5x - \frac{2x^3}{3} + c\)
We know that \((3, 5)\) is a point on the curve, so \(f(3) = 5\).
Substitute \(x = 3\) and \(f(x) = 5\) into the equation:
\(5 = 5(3) - \frac{2(3)^3}{3} + c\)
\(5 = 15 - \frac{54}{3} + c\)
\(5 = 15 - 18 + c\)
\(5 = -3 + c\)
\(c = 8\)
Thus, \(f(x) = 5x - \frac{2x^3}{3} + 8\).
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