Hyperbola Question 2

Question 2 - 25 January - Shift 1

For some $a, b, c \in \mathbb{N}$, let $f(x)=a x-3$ and $g(x)=x^{b}+c, x \in \mathbb{R}$. If $(f \circ g)^{-1}(x)=(\frac{x-7}{2})^{1 / 3}$ then $(fog)(ac)+(gof)(b)$ is equal to __________

Show Answer

Answer: 2039

Solution:

Formula: Composition of functions

Let $fog(x)=h(x)$

$\Rightarrow h^{-1}(x)=(\frac{x-7}{2})^{\frac{1}{3}}$

$\Rightarrow h(x)=fog(x)=2 x^{3}+7$

$f \circ g(x)=a(x^{b}+c)-3$

$\Rightarrow a=2, b=3, c=5$

$\Rightarrow f \circ g(a c)=f \circ g(10)=2007$

$g(f(x)=(2 x-3)^{3}+5.$

$\Rightarrow gof(b)=gof(3)=32$

$\therefore (fog)(ac)+(gof)(b) = 2039$