Question

A rectangle has dimensions x  y.
 Express its area A as a function of x and its perimeter P

Answer 1

(x,y) coordinate
since f(x)=y
(x,f(x)) is the same coordinate but now as a function of x
a rectangel the sides must be equal so x will be same length as y, then x times y becomes x times x so it's 2 times x. f(x)=2x will be the surface as a function of x
i dont know what is meant with perimiter P

Answer 2

A=xy
P=2x+2y
now rearrange the expression for the perimeter
y=(P/2)-x
so,
\[A=x(\frac{P}{2}-x)\]