SOLUTION: Show that the area of the Norman window is a maximum when both r and x are equal to P(pie+4) *assume that the perimeter of the window is P So I figured that the r is the radius a

Algebra ->  Customizable Word Problem Solvers  -> Geometry -> SOLUTION: Show that the area of the Norman window is a maximum when both r and x are equal to P(pie+4) *assume that the perimeter of the window is P So I figured that the r is the radius a      Log On

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Question 927974: Show that the area of the Norman window is a maximum when both r and x are equal to P(pie+4)
*assume that the perimeter of the window is P
So I figured that the r is the radius and the x is the length
So I made this equation x+r = P(pie+4)

Answer by KMST(5328) About Me  (Show Source):
You can put this solution on YOUR website!
I think you have a typo. Both r and x are parts of a sum of lengths that is the perimeter, P , so they cannot be P%28pi%2B4%29%3EP .
Here is your Norman window:
P=2x%2B2r%2Bpi%2Ar--->P=2x%2B%282%2Bpi%29r and area=2xr%2Bpi%2Ar%5E2%2F2
If we have a fixed perimeter, the maximum area will be obtained when
x=r=P%2F%28pi%2B4%29 , and here is my proof:
P=2x%2B%282%2Bpi%29r<--->P-%282%2Bpi%29r=2x<--->x=P%2F2-%282%2Bpi%29r%2F2
Substituting into area=2xr%2Bpi%2Ar%5E2%2F2 , we get
area=%28P-%282%2Bpi%29r%29r%2Bpi%2Ar%5E2%2F2
area=Pr-%282%2Bpi%29r%5E2%2Bpi%2Ar%5E2%2F2
area=Pr-2r%5E2-pi%2Ar%5E2%2B%28pi%2F2%29%2Ar%5E2
area=Pr-2r%5E2-%28pi%2F2%29%2Ar%5E2
area=Pr-%282%2Bpi%2F2%29%2Ar%5E2
area=-%28%284%2Bpi%29%2F2%29%2Ar%5E2%2BPr
area is a quadratic function of r
A quadratic function like f%28x%29-ax%5E2%2Bbx%2Bc has a maximum if the leading coefficient (the a in the term with the square) is negative.
The maximum happens when the variable has the value x=-b%2F2a
In this case, the maximum happens when
r=-P%2F%282%28-%284%2Bpi%29%2F2%29%29%29=P%2F%284%2Bpi%29=P%2F%28pi%2B4%29
Substituting r=P%2F%28pi%2B4%29 into P-%282%2Bpi%29r=2x , we get
P-%282%2Bpi%29%28P%2F%28pi%2B4%29%29=2x
P-%282%2Bpi%29P%2F%28pi%2B4%29=2x
P%281-%282%2Bpi%29%2F%28pi%2B4%29%29=2x
P%28pi%2B4-%282%2Bpi%29%29%2F%28pi%2B4%29=2x
P%28pi%2B4-2-pi%29%2F%28pi%2B4%29=2x
2P%2F%28pi%2B4%29=2x--->x=P%2F%28pi%2B4%29