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Calculus/calculus max/min problems

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Question
There are 2 problems that i can not seem to figure out.  
1. Find the dimensions of the rectangle of maximum area that can be inscribed in a semicircle or radius a, if two vertices lie on the diameter.
2. A commercial cattle rance currently allows 20 steers per acre of grazing land; on the average its steers weigh 2000lb at market.  Estimates by the Agriculture Department indicate that the average market weight per steer will be reduced by 50lb for each additional steer added per acre of grazing land.  How many steers per acre should be allowed in order for the ranch to get the largest possible total market weight for its cattle?
Thank you so much.

Answer

drawing
1.
As Its drawn in the image I attached, we conclude that :
y=(a^2-x^2)^(1/2). The area of the rectangle is S=2x*y whic is
2x*(a^2-x^2)^(1/2). We need to maximize this function, so we derive & set the derivative equal zero:
S'(x)=2(a^2-x^2)^(1/2)-2x*2x/2(a^2-x^2)^(1/2).
S'(x)=0 -> 2(a^2-x^2)^(1/2)=4x^2/2(a^2-x^2)^(1/2).
(a^2-x^2)=x^2 -> 2x^2=a^2 -> x=a/2.

As for 2, I didn't understand the problem well .. please try to explain it again. Thank you

Alon.

Alon Mandes

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