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Using the inverse composition rule you know that if (fog)=x and (gof)=x then the functions are inverses.

( g o f ) = ( 7 x + 1 ) − 1 7  this will result in (gof)=x ( f o g) = 7 x − 1 7 +1 = x − 1 + 1 =x

graphically you can check this if it seems that the functions are reflected over the line x=y.

Step-by-step explanation:

a 4/6 = 9/12   not proportional

step-by-step explanation:

to find which are not proportional, we can use cross products

if it is true, they are proportional.   if it is not true, they are not proportional.

a 4/6 = 9/12

4* 12 = 6*9

48 = 54

not this is not proportional  

b 2/3 = 12/18  

2* 18 = 3 * 12

36 = 36

this is proportional

c 5/4 = 10/8  

5*8 = 4*10

40 = 40

this is proportional

d 7/5 = 35/25

7*25 = 5 * 35

175 = 175

is proportional