Part 1
f(x) + g(x)
3x+2 + 4x
(3x+4x) + 2
7x+2
Therefore, f(x)+g(x) = 7x+2
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Part 2
f(x) = 3x+2
f( g(x) ) = 3( g(x) ) + 2 ... replace every x with g(x)
f( g(x) ) = 3(4x) + 2 .... plug in g(x) = 4x
f( g(x) ) = 12x + 2
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Part 3
Let's say x is the number of minutes.
Furthermore, let's say f(x) and g(x) are phone plan costs. Computing f(x)+g(x) will give the total cost of both plans combined. This is one example where it's useful to add the functions.
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As for a second example, let's say g(x) = 4x represents the total cost to fill up your car when you use x gallons of gas. The 4 out front means gas is $4 per gallon.
When we compute f( g(x) ) = 3*g(x) + 2, we could have 3 different cars that use the same amount of gas (x). The +2 at the end could be something like we have to pay $2 for parking.
Overall, f( g(x) ) = 12x + 2 is the total cost of filling up 3 cars and paying for the extra $2 parking.
Notice the output of g(x) is fed directly as the input of f(x). Think of it like a chain reaction.
This is one example of many where function composition is useful.