MCQ
If $3\text{f(x)}+5\text{f}\Big(\frac{1}{\text{x}}\Big)=\frac{1}{\text{x}}-3$ for all non-zero x, then f(x) =
  • A
    $\frac{1}{14}\Big(\frac{3}{\text{x}}+5\text{x}-6\Big)$
  • B
    $\frac{1}{14}\Big(-\frac{3}{\text{x}}+5\text{x}-6\Big)$
  • C
    $\frac{1}{14}\Big(-\frac{3}{\text{x}}+5\text{x}+6\Big)$
  • D
    None os these.

Answer

  1. None os these.

Solution:

$3\text{f(x)}+5\text{f}\Big(\frac{1}{\text{x}}\Big)=\frac{1}{\text{x}}-3\ ...(\text{i})$

Multiplying (1) by 3,

$15\text{f}\Big(\frac{1}{\text{x}}\Big)+9\text{f(x)}=\frac{3}{\text{x}}-9\ ...(\text{ii})$

Replacing x by $\frac{1}{\text{x}}$ in (i)

$3\text{f}\Big(\frac{1}{\text{x}}\Big)+5\text{f(x)}=\text{x}-3$

Multiplying by 5

$15\text{f}\Big(\frac{1}{\text{x}}\Big)+25\text{f(x)}=5\text{x}-15\ ...(\text{iii})$

Solving (ii) and (iii),

$-16\text{f(x)}=\frac{3}{\text{x}}-5\text{x}+6$

$\Rightarrow\text{f(x)}=\frac{1}{16}\Big(-\frac{3}{\text{x}}+5\text{x}-6\Big)$

Disclaimer: The question in the book has some error, so, none of the options are matching with the solution. The solution is created according to the question given in the book.

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