Let $f(x)$ be a polynomial of degree three satisfying $f(0)=-1$ and $f(1)=0$. Also, 0 is a stationary point of $f(x)$. If $f(x)$ does not have an extremum at $x=0$, then the value of the integral $\int \frac{f(x)}{x^3-1} d x$, is
Answer & explanation
Correct answer: option 2
Let $f(x)=a x^3+b x^2+c x+d$. Then,
$f(0)=-1$ and $f(1)=0$
$\Rightarrow d=-1$ and $a+b+c+d=0$
$\Rightarrow d=-1$ and $a+b+c=1$ ... (i)
It is given that x = 0 is a stationary point of f(x) but it is not a point of extremum. Therefore,
$f'(0)=0, f''(0)=0$ and $f'''(0) \neq 0$
Now,
$f(x)=a x^3+b x^2+c x+d$
$\Rightarrow f'(x)=3 a x^2+2 b x+c, f''(x)=6 a x+b$ and $f'''(x)=6 a$
∴ $f'(0)=0, f''(0)=0$ and $f'''(0) \neq 0$
$\Rightarrow c=0, b=0$ and $a \neq 0$ ....(ii)
From (i) and (ii), we get
$a=1, b=c=0$ and $d=-1$
∴ $f(x)=x^3-1$
Hence, $\int \frac{f(x)}{x^3-1} d x=\int 1 . d x=x+C$