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If $f(x) = x^{2} – 7x$ and $g(x) = x + 3,$ then the minimum value of $f(g(x)) – 3x$ is

  1. $ -16$
  2. $ -15$
  3. $ -20$
  4. $ -12$
in Quantitative Aptitude 2.7k points 5 79 342
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1 Answer

1 vote

Given that,

  • $f(x) = x^{2}-7x$
  • $g(x) = x+3$

Now, $f(g(x))-3x = f(x+3)-3x = (x+3)^{2} – 7(x+3)-3x$

$\qquad \qquad \qquad = x^{2}+9+6x-7x-21-3x = x^{2}-4x-12$

Let $h(x) = x^{2}-4x-12$

$\Rightarrow h’(x) = 2x-4$

For minimum value of $x:$

$\Rightarrow h’(x) = 0.$

$\Rightarrow 2x-4=0$

$\Rightarrow \boxed{x=2}$

Now, $h(2) = 2^{2}-4(2)-12$

$\Rightarrow h(2)=4-8-12.$

$\Rightarrow \boxed{h(2) = -16}$

$\therefore$ The minimum possible value of $f(g(x))-3x$ is $-16.$

Correct Answer $:\text{A}$

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