Question 575890
A rain gutter is to be made of a sheet that is 10in wide by turning up the edges 90 degrees.
:
Using the depth as x, 
then 
(10-2x) = the width of the gutter
; 
y = the cross sectional area
:
y = depth * width
y = x(10-2x)
y = -2x^2 + 10x
or
f(x) = -2x^2 + 2x
:
Graphing this would make it clear
{{{ graph( 300, 200, -2, 8, -10, 20, -2x^2+10x) }}}
You can see the domain is >0 to < 5, with max area when x= 2.5