Real Function of Two Variables/Examples/y by Root of 1 minus x^2
Jump to navigation
Jump to search
Examples of Real Functions of Two Variables
Let $z$ denote the function defined as:
- $z = y \sqrt {1 - x^2}$
The domain of $z$ is:
- $\Dom z = \closedint {-1} 1 \times \R$
Proof
The domain of $z$ is given implicitly and conventionally.
What is meant is:
- $z: S \to \R$ is the function defined on the largest possible subset $S$ of $\R^2$ such that:
- $\forall \tuple {x, y} \in S: \map z {x, y} = y \sqrt {1 - x^2}$
From Domain of Real Square Root Function, in order for the real square root function to be defined, its argument must be non-negative.
Hence for $z$ to be defined, it is necessary for:
\(\ds 1 - x^2\) | \(\ge\) | \(\ds 0\) | ||||||||||||
\(\ds \leadsto \ \ \) | \(\ds x^2\) | \(\le\) | \(\ds 1\) | |||||||||||
\(\ds \leadsto \ \ \) | \(\ds \size x\) | \(\le\) | \(\ds 1\) |
So for $z$ to be defined, it must be that $-1 \le x \le 1$.
There are no such restrictions on the value of $y$.
Hence the result.
$\blacksquare$
Sources
- 1963: Morris Tenenbaum and Harry Pollard: Ordinary Differential Equations ... (previous) ... (next): Chapter $1$: Basic Concepts: Lesson $2 \text C$: Function of Two Independent Variables