Work:
First it's a rectangle so its area is equal to the product (multiplication) of both sides of the rectangle (dimensions). A = a x b = ab
NOW: knowing the the circumference or perimeter is equal to 400m, we can say that P = 400 = 2(a+b) = 2a + 2b since the given polynom is rectangle. 2a + 2b = 400 <==> 2a = 400 - 2b <==> a = 200 - b.
We gave an expression of a in function of b. Now we can replace the variable a by 200 - b in the first expression of the area.
A = ab = (200-b)b = 200b-b^2 = -b^2 + 200b
A is now a quadratic equation. We note A(b) the epression -b^2 +200b so:
A(b) = -b^2 +200b
We can already see that A is a quadratic equation of the form:
ax^2 + b + c. The a coefficient is negative which will lead to closed parabola when looking from the top. Now we need to find the maximum of the function by using the derivatives:
A(b) = -b^2 +200b <==> A'(b) = -2b + 200b
and -2b + 200b = 0 <==> b = 100;
So the derivative function crosses the x-axis at (100; 0).
So is increasing over ]-∞; 100] and decreasing over [100; ∞[.
We obtain a maxima on (100; x) with A. To find it we need to replace 100 by b in the function A.
A(b) = -b^2 + 200b
<==> A(100) = -100^2 + 200 x 100 = - 10000 + 20000 = 10000
Now let's find a: if b = 100 what equals a ?
We know that ab = 10000 <==> 100a = 10000 <==> a = 100;
And we verify that ab = 100 x 100 = 10000.
The rectangle needs to be a square to reach the maximum area of 10000m^2 or also 0.01 km^2.
Q.E.D.
