CORE 3 Composite functions

Hey...
I have this question:
The function f is defined by f(x) = | x – 2 | – 3

g(x) = x^2 – 4x + 11

Question - Find gf(–1).

I was wondering, for this question i got:

| x^2 - 4x + 11 - 2| - 3

When substituting in the value of x as -1, why do you not need to consider the positive and negative values i.e. + (x^2 - 4x + 11 - 2) - 3
and -(x^2 -4x + 11 - 2) - 3 ??

Reply 1

DarkSenrine

14

Forgive me if I'm wrong, but isn't that f o g?

Reply 2

haloturk

11

Original post by stanmoor

Hey...
I have this question:
The function f is defined by f(x) = | x – 2 | – 3

g(x) = x^2 – 4x + 11

Question - Find gf(–1).

I was wondering, for this question i got:

| x^2 - 4x + 11 - 2| - 3

When substituting in the value of x as -1, why do you not need to consider the positive and negative values i.e. + (x^2 - 4x + 11 - 2) - 3
and -(x^2 -4x + 11 - 2) - 3 ??

You have to put f into g :
Actually you could even do

F(-1) first and whet ever that equals put that in for x in g(x)

Reply 3

jimimick

2

hmmm i got (|x-2|-3)^2 - 4(|x-2|-3) +11

so when x= -1

0 - 0 + 11

answer is 11? i dunno tbh, gd luck

(edited 13 years ago)

Reply 4

alexs2602

22

Original post by stanmoor

Hey...
I have this question:
The function f is defined by f(x) = | x – 2 | – 3

g(x) = x^2 – 4x + 11

Question - Find gf(–1).

I was wondering, for this question i got:

| x^2 - 4x + 11 - 2| - 3

When substituting in the value of x as -1, why do you not need to consider the positive and negative values i.e. + (x^2 - 4x + 11 - 2) - 3
and -(x^2 -4x + 11 - 2) - 3 ??

Just go the ****ing easy way

f(-1) = |-1 - 2| - 3 = 0

g(0) = 0^2 - 4(0) +11 = 11

gf(-1) = 11

Reply 5

Coda

16

Original post by stanmoor

Hey...
I have this question:
The function f is defined by f(x) = | x – 2 | – 3

g(x) = x^2 – 4x + 11

Question - Find gf(–1).

I was wondering, for this question i got:

| x^2 - 4x + 11 - 2| - 3

When substituting in the value of x as -1, why do you not need to consider the positive and negative values i.e. + (x^2 - 4x + 11 - 2) - 3
and -(x^2 -4x + 11 - 2) - 3 ??

The answer is 11. You do f(-1) which is | -3 | - 3 (Modulus of -3 is 3, so 3 - 3)
then you do g(0) =

0^2 - 4(0) + 11

= 11.

Reply 6

viksta1000

15

Original post by stanmoor

Hey...
I have this question:
The function f is defined by f(x) = | x – 2 | – 3

g(x) = x^2 – 4x + 11

Question - Find gf(–1).

I was wondering, for this question i got:

| x^2 - 4x + 11 - 2| - 3

When substituting in the value of x as -1, why do you not need to consider the positive and negative values i.e. + (x^2 - 4x + 11 - 2) - 3
and -(x^2 -4x + 11 - 2) - 3 ??

the people who are giving 11 as the final answer are right i believe

the reason you dont consider the positive AND negative values is because you square the modulus which will always give a positive answer (negative squared = positive)

Reply 7

alexs2602

22

Sorry, are people unable to read here? I gave the answer in a clear form and jimimick got it before me but did it in a frankly awkward way.

Reply 8

Chelle-belle

13

Original post by alexs2602

Sorry, are people unable to read here? I gave the answer in a clear form and jimimick got it before me but did it in a frankly awkward way.

I don't really understand what you are trying to say. Do you really mean no one can post after you even if they think they might contribute a bit to OP's understanding of the topic? No one gave you negative rep nor said anything negative about you.

Reply 9

alexs2602

22

Original post by Chelle-belle

I don't really understand what you are trying to say. Do you really mean no one can post after you even if they think they might contribute a bit to OP's understanding of the topic? No one gave you negative rep nor said anything negative about you.

I'm saying; "why answer a question twice?"

Plus it feels like I'm being ignored as you've answered the same as what I've said twice in the last hour; on this thread and stanmoor's log thread.

Reply 10

Chelle-belle

13

Original post by alexs2602

I'm saying; "why answer a question twice?"

Plus it feels like I'm being ignored as you've answered the same as what I've said twice in the last hour; on this thread and stanmoor's log thread.

Maybe your posts were inadequate or people still felt they could contribute. Furthermore, why not answer a question twice (if it is the first time for the one answering it) if you have nothing else to do, and come to this forum specifically to answer questions, anyway? I could just as easily ask you why you bothered to come back to this thread, and the other thread, and probably a few other threads, for no apparent reason, and why you bothered to post something like this.

And I probably did ignore you on the other thread, not that you should take offense to that, since a) I ignore most posts without some sort of actual explanation (which is why I later deleted my post); b) it shouldn't really affect you directly.

(edited 13 years ago)

Reply 11

alexs2602

22

Original post by Chelle-belle

Maybe your posts were inadequate or people still felt they could contribute. Furthermore, why not answer a question twice (if it is the first time for the one answering it) if you have nothing else to do, and come to this forum specifically to answer questions, anyway?