Help! (maths)

touch · 16 Jan 2007 at 15:46

I need to find the inverse of a function.
Its been a good year or two since i did my higher maths and im not sure i remember it correctly.

f(x) = 4x + 3

so the inverse is 1/4x -3 ?

Visage · 16 Jan 2007 at 15:55

touch said:
I need to find the inverse of a function.
Its been a good year or two since i did my higher maths and im not sure i remember it correctly.

f(x) = 4x + 3

so the inverse is 1/4x -3 ?

if the inverse of f(x) is f'(x) then f(f'(x)) = x.

So, to check your example:

4(1/4x-3) + 3 = 1/x -12 + 3 = 1/x -9.

So your answer is wrong.

EDIT: I've assumed that by 1/4x you mean 1 divided by 4x, as opposed to x divided by 4. Either way the answer is incorrect, though...

Dave · 16 Jan 2007 at 16:03

Using the "interchanging variables" method: (swap variables around and solve for y)

y = 4x + 3

Changing variables:

x = 4y + 3
(x - 3) / 4 = y

Therefore f^-1(x) = (x - 3) / 4

Test: if x = 2, f(x) = 11
f^-1(11) = 2

Mackass · 18 Jan 2007 at 01:49

Don't want to start a new thread.

But basically, what is that symbol {x E R}? The topic is functions, anyone know?

Jotun · 18 Jan 2007 at 01:58

Mackass said:
Don't want to start a new thread.

But basically, what is that symbol {x E R}? The topic is functions, anyone know?

I believe it means x is all real numbers.

Mackass · 18 Jan 2007 at 02:00

Jotun said:
I believe it means x is all real numbers.

Ah right, I get it now, cheers.

titaniumx3 · 18 Jan 2007 at 02:03

the 'e' symbols should be read as 'element of'.