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BITSAT 2015
Exam
BITSAT
Year
2015
Subject
Mathematics
Chapter
Application of Derivatives

BITSAT 2015 · Mathematics · Application of Derivatives

Let f(x) be a polynomial of degree three satisfying f(0)=-1 and...

Let f(x)f(x) be a polynomial of degree three satisfying f(0)=−1\mathrm{f}(0)=-1 and f(1)=0.Also,0\mathrm{f}(1)=0. \mathrm{Also}, 0 is a stationary point of f(x)f(x) If f(x)f(x) does not have an extremum at x=0x=0, then the value of ∫f(x)x3−1dx\int \frac{f(x)}{x^{3}-1} d x is

Options

  1. Ax22+C\frac{x^{2}}{2}+C
  2. Bx+Cx+C
  3. Cx36+C\frac{x^{3}}{6}+C
  4. DNone of these

Correct answer

B. x+Cx+C

Step-by-step solution

Let f(x)=ax3+bx2+cx+df(x)=a x^{3}+b x^{2}+c x+d Put x=0andx=1x=0 and x=1 Then, we get f(0)=-1 and f(1)=0⇒d=−1f(1)=0\Rightarrow \mathrm{d}=-1 and a+b+c+d=0⇒a+b+c=1\mathrm{a}+\mathrm{b}+\mathrm{c}+\mathrm{d}=0\Rightarrow \mathrm{a}+\mathrm{b}+\mathrm{c}=1 It is given that x=0x=0 is a stationary point of f(x)\mathrm{f}(\mathrm{x}), but it is not a point of extremum. Therefore, f(0)=0=f′′(0)andf′′′(0)=0f(0)=0=f^{\prime \prime}(0) and f^{\prime \prime \prime}(0)=0 Now, f(x)=ax3+bx2+cx+d⇒f(x)=3ax2+2bx+c\mathrm{f}(\mathrm{x})=\mathrm{ax}^{3}+\mathrm{bx}^{2}+\mathrm{cx}+\mathrm{d}\Rightarrow f(x)=3 a x^{2}+2 b x+cf′′(x)=6ax+2b and f′′(x)=6af′=0,f′′(0)=0 and f′′′(0)=0≠0\begin{array}{l} f^{\prime \prime}(x)=6 a x+2 b \text{ and } f^{\prime \prime}(x)=6 a \\ f^{\prime}=0, f^{\prime \prime}(0)=0\text{ and } f^{\prime \prime \prime}(0)=0 ≠ 0 \end{array}⇒c=0,b=0anda≠0From\Rightarrow c=0, b=0 and a \neq 0 FromEqs. (i) and (ii), we get \mathrm{a}=1,,\mathrm{ b}=\mathrm{c}=0andand\mathrm{d}=−1Putthesevaluesin-1Put these values in\mathrm{f}((\mathrm{x})wegetwe getf(x)=x^{3}-1Hence,Hence,\int \frac{\mathrm{f}((\mathrm{x})}{\mathrmx^{3}-1}\mathrm{dx}=\int \frac{\mathrmx^{3}-1}{\mathrmx^{3}-1} \mathrm{ d} \mathrm{x}=\int 1 \mathrm{ d} x=x\mathrm{x}=\mathrm{x}+\mathrm{C}$
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