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**denis_gylaev****Member**- Registered: 2015-03-19
- Posts: 66

Hello, I do not know the answers to these problems.

1)

2)

3)

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**bob bundy****Administrator**- Registered: 2010-06-20
- Posts: 8,054

hi denis_gylaev

Q1) f o g means apply g and then f to the result:

f o g (x) = 2.(3x + c) + 7

Do a similar thing for g o f and set them equal.

Q2) This will get the equation of the line AB:

By substituting the known values of x and y you can get equations to find a and b.

Q3) Just set f and g equal and solve for x.

Bob

Children are not defined by school ...........The Fonz

You cannot teach a man anything; you can only help him find it within himself..........Galileo Galilei

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**denis_gylaev****Member**- Registered: 2015-03-19
- Posts: 66

Hi Bob, thank you so much for the help.

For Q2, I got (-1,-2) and (1,2) and for Q3 I got (-1, 3)

However, I still do not understand what you mean for number 1. Please advise.

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**bob bundy****Administrator**- Registered: 2010-06-20
- Posts: 8,054

hi denis_gylaev

Think of a function as being a box into which you can input a number and another number is output. f and g are two such functions.

If you join the output from g to the input for f we get a composite function. To find out what single function is the same as the composite function you have to apply first g (which means you have an expression in x and put that through the second box for f.

g(x) = 3x + c so the box takes a number, times it by 3 and then adds c. If that is the input for the f box the output from f looks like this:

2(3x + c) + 7 The input is (3x + c) and this is times by 2 and 7 added.

So the composite function for (f o g) is 2(3x + c) + 7. You can simplify this yourself.

You might think at first that g o f will give the same result but it doesn't. First do f ... 2x + 7 and then make this the input into g ... 3(2x + 7) + c

We want these to be equal so just make an equation by putting them equal to each other. The x terms cancel out (why?) so you're left with an equation for c.

There is a MIF page on this here: http://www.mathsisfun.com/sets/function … ition.html

Note The x comes after the function letter [eg. f(x) not (x) f ] so when you want to apply a second function you have to put it before the first. eg g(f(x)) means apply f then g to the result. This can be confusing but think about log(sin(x)) for example. o is used to indicate that we are combining functions.

Hope that helps,

Bob

Children are not defined by school ...........The Fonz

You cannot teach a man anything; you can only help him find it within himself..........Galileo Galilei

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