Funky functions

Algebra Level 2

Suppose f ( x ) = 3 x 3 x 2 f(x) = \frac{3-x}{3x-2} and g ( x ) = x + 1 2 x 4 g(x)= \frac{x+1}{2x-4} . Find the value of m m for which f g ( m ) = 1 4 fg(m) = \frac{1}{4} .

Note: f g ( x ) = f ( g ( x ) ) fg(x) = f(g(x))


The answer is 3.

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1 solution

Noel Lo
May 10, 2015

Method 1:

f g ( m ) = f ( m + 1 2 m 4 ) = 3 m + 1 2 m 4 3 ( m + 1 2 m 4 ) 2 = ( 6 m 12 ) ( m + 1 ) 3 m + 3 ( 4 m 8 ) = 5 m 13 11 m fg(m) = f(\frac{m+1}{2m-4}) = \frac{3-\frac{m+1}{2m-4}}{3(\frac{m+1}{2m-4}) - 2} = \frac{(6m - 12)-(m+1)}{3m+3 - (4m - 8)} = \frac{5m-13}{11-m}

5 m 13 11 m = 1 4 \frac{5m-13}{11-m} = \frac{1}{4}

4 ( 5 m 13 ) = 11 m 4(5m-13) = 11-m

20 m 52 = 11 m 20m-52=11-m

( 20 + 1 ) m = 52 + 11 (20+1)m = 52+11

21 m = 63 21m = 63

m = 63 21 = 3 m=\frac{63}{21} = \boxed{3}

Method 2:

f g ( m ) = 1 4 fg(m) = \frac{1}{4}

3 g ( m ) 3 g ( m ) 2 = 1 4 \frac{3-g(m)}{3g(m) - 2} = \frac{1}{4}

12 4 g ( m ) = 3 g ( m ) 2 12-4g(m) = 3g(m)-2

7 g ( m ) = 14 7g(m) = 14

g ( m ) = 2 g(m) = 2

m + 1 2 m 4 = 2 \frac{m+1}{2m-4} = 2

m + 1 = 4 m 8 m+1 = 4m - 8

3 m = 9 3m = 9

m = 3 m = \boxed{3}

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