# Domain of freehand shape

kathrynp shared this question 4 years ago

This may be more a mathematical problem - should the domain of f(abs(x)) be the same as the domain of f(x)? GeoGebra seems to treat it as the reflection of f(x), x>0, in the y axis. Do I need to restrict the domain somehow? Very rusty at the moment!  1

obviously not: log(x) and log(|x|) have differents domain ; domain(f) and domain(|f|) are the same

try if[isdefined[f(x)],f(abs(x))] 1

i tried isdefined and it does not work fine

try if[f(x)==f(x),f(abs(x))]

it is my old vdersion of isdefined 1

I'm trying to work out how to tell my students that the answer they have given, which is the same as GeoGebra, is not the answer given by the exam board (it's a UK A Level (pre-university) question.) Reading the question again, I think it is ambiguous! Thanks for your solution, mathmagic - although I don't quite follow the syntax!) 1

Hi, gof (x) is defined <=> f(x) is defined and y=f(x) is a number that g(y) is defined.

In this case, f(x)=abs(x) is defined for all real and with y=abs(x), f(y) is defined when x or -x is in domain(f) so for me, GeoGebra is right.

For the syntax : f(x)==f(x) seems a boolean function true when f(x) is defined and false if not,.

If[<boolean function>,<function>] seems the function with the domain where <boolean function>==true

Cheers. 1

Where's the question from (Exam board / paper / year)? 1

Edexcel C3 Jan 2008 - no mention of domain in question, mark scheme or examiners' report, only implication in answer that it is restricted. 1

Mark scheme says "For the purpose of marking this paper, the graph is identical to (a)" which is not very helpful for students (but it is a mark scheme, not a model answer :) )

This is correct (as is GeoGebra :) )