Evaluating Composition

Algebra Level 1

f f and g g are functions defined as f ( x ) = 5 x + 6 2 x 2 f(x)= 5x + \frac{6}{2x-2} and g ( x ) = 4 x 3 x 2 g(x) = \frac{4x}{3x-2} . What is the value of ( f g ) ( 4 ) \left(f\circ g\right)(4) ?

Note: f g f \circ g denotes the composition of the 2 functions.


The answer is 13.

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5 solutions

Anish Shah
Dec 2, 2013

( f o g ) ( x ) = f ( g ( x ) ) ( f o g)(x) = f(g(x))

g ( 4 ) = 8 5 g(4) = \frac{8}{5}

f ( 8 5 ) = 8 + 5 = 13 f(\frac{8}{5}) = 8 + 5 = 13

thnx

somesh shehan - 7 years, 6 months ago
Adarsh Pathak
Dec 1, 2013

We've the values of f(x) & g(x): Now, we got to solve f o g ( 4 ) f o g (4) , If we simplify this term a li'l bit, we get: f ( g ( x ) ) f(g(x)) where x=4 .

First Step: Solve the g(x) for x=4 : 4 × 4 3 × 4 2 = 8 5 \frac{4 \times 4}{3 \times 4 - 2} = \frac{8}{5}

Second Step: solve f(x) for x=8/5 : 5 × 8 5 + 6 2 × 8 5 2 5 \times \frac{8}{5} + \frac{6}{2 \times \frac{8}{5} -2}

on further solving we get: 8 + 5 = 13. 8+5=13.

So, our answer is 13.

can we substitute 4 after substituting the g(x) in f(x)

Vishal Chilur G - 7 years, 6 months ago

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Yes :)

Anish Shah - 7 years, 6 months ago

thnx

Muhammad Talha - 7 years, 6 months ago
Abubakarr Yillah
Jan 6, 2014

g ( x ) = 4 x 3 x 2 g({x})=\frac{4x}{3x-2}

g ( 4 ) = 4 ( 4 ) 3 ( 4 ) 2 g({4})=\frac{4({4})}{3({4})-2}

which simplifies to 8 5 \frac{8}{5}

f ( x ) = 5 x + 6 2 x 2 f({x})={5x}+\frac{6}{2x-2}

f ( 8 5 ) = 5 ( 8 5 ) + 6 2 ( 8 5 ) 2 f(\frac{8}{5})={5}(\frac{8}{5})+\frac{6}{2(\frac{8}{5})-2}

which simplifies to 8 + 5 {8}+{5}

that gives an answer of 13 \boxed{13}

Andre Yudhistika
Jan 4, 2014

fog(4)=f(g(4))

g(4)=4(4)/(3(4)-2)=8/5

f(8/5)=..... =13

Hugo Pinheiro
Dec 2, 2013

f belongs g so, we need to put all in the function g x f

Can you explain what you mean and show how this leads to a solution?

Calvin Lin Staff - 7 years, 6 months ago

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