Answer:
a*(20-a)
Explanation:
Let's assume that the length of the garden is a the width is b and the area is s.
b = 40/2 - a = 20-a
s =a*(20-a)=-a^2+20a=-(a-10)^2+100
then,
when a=10, max(-(a-10)^2) =0, max(s)=100
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