domain & co-domain

解釋下... 唔係好明

take y=f(x) as an example
Domain 就係the range of possible values of x
Co-domain 就係the range of the range of y

for example y = x^2
Domain 就係all real no
Co-domain 就係all real no

eg2 y = square root of x
domain 就係 all +ve no and zero (non-negative real no)
Codomain : all +ve real no

唔該曬-]

Strictly speaking, the co-domain is a set (or a group of numbers) which contains all the possible values of y, but not only the possible values of y, i.e. it could include values which would never be y. For example, for y = x², the co-domain is (or could be) the real numbers, but not all real numbers could be y. For example, y couldn't = -1, if we restrict our domain to real numbers (complex number i² = -1). In fact, y could only be non-negative real numbers, and that's called the range. The range includes only all the possible values of y. This is the summary of the above example:
Function: y = x²
Domain: real numbers
Co-domain: real numbers
Range: non-negative real numbers

你返學我比本書你望下啦.

Correction: The "range" I mentioned above is better called the "image", as the term "range" could refer to either the image or co-domain.