Question 889070
Let the side parallel to the river be X and the other side Y.
{{{X+X+Y=7200}}}
{{{2X+Y=7200}}}
The area would then be,
{{{A=XY}}}
Substitute for Y using the perimeter equation,
{{{Y=7200-2X}}}
{{{A=X(7200-2X)=7200X-2X^2}}}
To find the extrema, differentiate with respect to X and set the derivative equal to zero.
{{{dA/dX=7200-4X=0}}}
{{{X=7200/4}}}
{{{highlight(X=1800)}}}
Then,
{{{Y=7200-3600}}}
{{{highlight(Y=3600)}}}