MCQ
Let $f$ be a polynomial function such that $f(3x)\, = f'(x) , f''(x)$, for all $x \in R$. Then
  • A
    $f(2) + f'(2)\,= 28$
  • $f''(2) -f'(2)\, = 0$
  • C
    $f''(2)-f'(2)\,= 4$
  • D
    $f(2) -f'(2) + f''(2)\, = 10$

Answer

Correct option: B.
$f''(2) -f'(2)\, = 0$
b
Let $f\left( x \right) = a{x^3} + b{x^2} + cx + d$

$f\left( {3x} \right) = 27a{x^3} + 9b{x^2} + 3cx + d$

$f'\left( x \right) = 3a{x^2} + 2bx + c$

$f''\left( x \right) = 6ax + 2b$

$f\left( {3x} \right) = f'\left( x \right)f''\left( x \right)$

$27a = 18{a^2}$

$a = \frac{3}{2},b = 0,c = 0$

$f\left( x \right) = \frac{3}{2}{x^3}$

$f'\left( x \right) = \frac{9}{2}{x^2},f''\left( x \right) = 9x$

$f'\left( 2 \right) = 18$

$f''\left( 2 \right) = 18$

$ \Rightarrow f''\left( 2 \right) - f\left( 2 \right) = 0$

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