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Ritzie has 80 metres of fencing wire which she wants to use to fence a rectangular plot for gardening. She wished to get the right length and width such that the area enclosed is the maximum. Let x to represent length and y to represent width.
1) Write two equations one for perimetre and the other for area using X and Y
2)Obtain only one equation in terms of area from the equations in 1​



Answer :

reinhard10158

Step-by-step explanation:

1)

the perimeter P

P = 2x + 2y = 80 m

the area A

A = x × y

2)

2x + 2y = 80

2y = 80 - 2x

y = 40 - x

and we use that in the equation for area

A = x × (40 - x) = 40x - x²

we find the maximum (or minimum) of a function by finding the 0s of the first derivative.

A' = 40 - 2x = 0

40 = 2x

x = 20

y = 40 - x = 40 - 20 = 20

and just to be sure :

if the second derivative at that point x is negative, we have a maximum, if it is positive, we have a minimum.

A'' = -2

so, it is negative for all x incl. x = 20.

therefore, we have a maximum.

the rectangle with perimeter of 80 m and the max. area is a square of 20 m side length.

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