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# functions c3 watch

1. i hate functions

f(x) =x^2 D[0,4] R[0,16]
g(x) =x + 3 D(- infinity , infinity) R(- infinity, infinity)

gf(x) = x^2 + 3 D[0, 4] R[3, 19]

i get so confused with D and R for composites
if u have a good explanation or step then tell me cos every time i have to think hard about it
2. the reason im asking is cos another question is making me doubt the method i use to find range of composites.

the answer for this is D(0, infinity) R[-25, infinity) -> why is there even a square bracket when theres none in the domain????

usually to find fg(x) range, i use range of g(x) and put it into f(x) but the markscheme for question above, they put the domain of g(x) into f(x), which contradicts a lot of questions ive done which was marked right by the markscheme as well.
3. (Original post by ihatePE)
the reason im asking is cos another question is making me doubt the method i use to find range of composites.

the answer for this is D(0, infinity) R[-25, infinity) -> why is there even a square bracket when theres none in the domain????

usually to find fg(x) range, i use range of g(x) and put it into f(x) but the markscheme for question above, they put the domain of g(x) into f(x), which contradicts a lot of questions ive done which was marked right by the markscheme as well.
Your method sounds fine - maybe you misinterpreted the mark scheme?

What did you get for the range of g?
4. ihatePE By the way your first post wasn't really a question which is probably why you didn't get any responses. Please always post a question
5. (Original post by Notnek)
ihatePE By the way your first post wasn't really a question which is probably why you didn't get any responses. Please always post a question

find the domain and range of gf(x)

and they gave me these info f(x) =x^2 D[0,4]
g(x) =x + 3 D(- infinity , infinity)

i found the range for g and f.
then i found the gf(x) domain and function to be gf(x) = x^2 + 3 D[0, 4] R[3, 19]

my teacher marked this, it's right. ive done a few more similar questions but today i came acros this question

(Original post by ihatePE)

the answer for this is D(0, infinity) R[-25, infinity) -> why is there even a square bracket when theres none in the domain????

usually to find fg(x) range, i use range of g(x) and put it into f(x) but the markscheme for question above, they put the domain of g(x) into f(x), which contradicts a lot of questions ive done which was marked right by the markscheme as well.
and the whole finding range for composities suddenly contradicts, then theres the confsion of where the square brackets come from if theres no square bracket in domain
6. (Original post by ihatePE)
and the whole finding range for composities suddenly contradicts, then theres the confsion of where the square brackets come from if theres no square bracket in domain
Please answer the question I asked you:

What did you get for the range of g?
7. i probably need sleep, its so hard to think atm
8. (Original post by ihatePE)
i probably need sleep, its so hard to think atm
Okay. If you still need help tomorrow it will probably be best to start a new thread. And please post the whole question and all of your working. Otherwise it's hard to help you.

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