Answer:
The lengths of the sides are 20 cm and 20 cm
Step-by-step explanation:
Given
Perimeter, P = 80cm
Represent the length and width with L and W, respectively;
[tex]P= 2*(L + B)[/tex]
Substitute 80 for P
[tex]80 = 2 * (L + B)[/tex]
Divide through by 2
[tex]40 = L + B[/tex]
[tex]L + B = 40[/tex]
Make L the subject of formula
[tex]L = 40 - B[/tex]
Area of a rectangle is calculated as thus;
[tex]Area = L * B[/tex]
Substitute 40 - B for L
[tex]Area = (40 - B) * B[/tex]
Express this as a function
[tex]A(B) = (40 - B)* B[/tex]
[tex](40 - B)* B = A(B)[/tex]
Set A(B) = 0 to determine the roots
Hence;
[tex](40 - B)* B = 0[/tex]
[tex]40 - B = 0[/tex] or [tex]B = 0[/tex]
[tex]40 = B[/tex] or [tex]B = 0[/tex]
[tex]B = 40[/tex] or [tex]B = 0[/tex]
The maximum area of a rectangle occurs at half the sum of the roots;
So;
[tex]B= \frac{B_1 + B_2}{2}[/tex]
[tex]B= \frac{40+0}{2}[/tex]
[tex]B= \frac{40}{2}[/tex]
[tex]B = 20[/tex]
Recall that [tex]L = 40 - B[/tex]
[tex]L = 40 - 20[/tex]
[tex]L = 20[/tex]
Hence the lengths of the sides are 20 cm and 20 cm
