Question 1180587


If the length ={{{x}}} and the width ={{{y}}} then the perimeter 
{{{P=40}}}
{{{P=2x+2y=40}}}
→{{{x+y=20}}}
→{{{y=20-x}}}

As for the area A
{{{A=x*y}}}.........substitute {{{y}}}
{{{A=x(20-x)}}}
{{{A=20x-x^2}}}

And we have to find an extreme for that 
{{{A}}}'={{{20-2x}}} by setting the derivative to ={{{0}}}
{{{20x-2x=0}}}
{{{2(10-x)x=0}}}
{{{x=10cm}}}

then
→{{{y=20-x}}}
{{{y=10cm}}}