If f(x) = 5x - 7, solve for x: f[f(x)] = f(x)

[Guest]

How in the world do I do this:

1) If f(x) = 5x - 7, solve for x: f[f(x)] = f(x)

The answer is 5(5x - 7) - 7 = 5x - 7 but I don't understand?

2) If f(x) = 1/(2x) and g(x) = 1/(x + 1) then g(f(x)) = ?

The answer is 1/(1/2x) + 1 x 2x/2x = 2x / 1+2x

I'm so confused!

 

[skeeter]

Re: Radicals

How in the world do I do this:

1) f(x)=5x-7, solve for x: f[f(x)]=f(x)

The answer is 5(5x-7)-7 = 5x - 7 but I don't understand?

no ... f[f(x)] = f(5x-7) = 5(5x-7) - 7 = 25x - 35 - 7 = 25x - 42

2) If f(x) = 1/(2x) and g(x) = 1/(x+1) then g(f(x)) = ?

g[f(x)] = g[1/(2x)] = 1/[1/(2x) + 1] = 1/[1/(2x) + 2x/(2x)] = 1/[(1 + 2x)/(2x)] =
2x/(1 + 2x)

The answer is 1/(1/2x)+1 x 2x/2x = 2x / 1+2x

Im so confused!

 

[stapel]

1) If f(x) = 5x - 7, solve for x: f[f(x)] = f(x)

The answer is 5(5x - 7) - 7 = 5x - 7....

But "5(5x - 7) - 7 = 5x - 7" isn't "solved for x". Are you sure the instructions were to "solve for x", and not, say, "evaluate the following"...?

Thank you.

Eliz.

 

[Guest]

I think I get it now.
Thank you.