If \(f'\left( x \right) = \frac{{{x^2}}}{2} - kx + 1\), such that f(0) = 0 and f(3) = 15. Find the value of k.

Concept:

Integration is the inverse process of differentiation and therefore it is called anti-differentiation.

i.e If g (x) = f’(x) then \(\smallint g\left( x \right)\;dx = \smallint f'\left( x \right)\;dx = f\left( x \right) + C\)

\(\smallint \left( {f\left( x \right) + g\left( x \right)} \right)\;dx = \smallint f\left( x \right)\;dx + \smallint g\left( x \right)\;dx\)

\(\smallint a{x^n}\;dx = a \times \frac{{{x^{n\; + \;1}}}}{{n + 1}} + C\)

Calculation:

Given: \(f'\left( x \right) = \frac{{{x^2}}}{2} - kx + 1\), such that f(0) = 0 and f(3) = 15.

Now, by integrating f’(x), we get

\(\Rightarrow f\left( x \right) = \smallint f'\left( x \right)\;dx = \smallint \left( {\frac{{{x^2}}}{2} - kx + 1} \right)\;dx\)

\(\Rightarrow f\left( x \right) = \smallint \frac{{{x^2}}}{2}\;dx - k\smallint x\;dx + \;\smallint dx\)

\(\Rightarrow f\left( x \right) = \frac{{{x^3}}}{6} - k \times \frac{{{x^2}}}{2} + x + C\)

As it is given that, f(0) = 0 and f(3) = 15.

\(\Rightarrow f\left( 0 \right) = C = 0\)

\(\Rightarrow f\left( x \right) = \frac{{{x^3}}}{6} - k \times \frac{{{x^2}}}{2} + x\)

\(\Rightarrow f\left( 3 \right) = \frac{9}{2} - \frac{9}{2}\;k + 3 = 15\)