A total of x feet of fencing is to form three sides of a level rectangular yard. What is the (继续) maximum possible area of the yard, in terms of x?(A)x^2/9(B)x^2/8(C)x^2/4(D)x^2(E)2x^2选B.怎么做?谢谢!

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A total of x feet of fencing is to form three sides of a level rectangular yard. What is the  (继续) maximum possible area of the yard, in terms of x?(A)x^2/9(B)x^2/8(C)x^2/4(D)x^2(E)2x^2选B.怎么做?谢谢!
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A total of x feet of fencing is to form three sides of a level rectangular yard. What is the (继续) maximum possible area of the yard, in terms of x?(A)x^2/9(B)x^2/8(C)x^2/4(D)x^2(E)2x^2选B.怎么做?谢谢!
A total of x feet of fencing is to form three sides of a level rectangular yard. What is the (继续) maximum possible area of the yard, in terms of x?
(A)x^2/9(B)x^2/8(C)x^2/4(D)x^2(E)2x^2
选B.怎么做?谢谢!

A total of x feet of fencing is to form three sides of a level rectangular yard. What is the (继续) maximum possible area of the yard, in terms of x?(A)x^2/9(B)x^2/8(C)x^2/4(D)x^2(E)2x^2选B.怎么做?谢谢!
设中间那个边为a,两边为b
有a+2b=x,x是常数,a=x-2b
有S=ab,即S=(x-2b)b=-2(b^2-0.5xb+(0.25x)^2)+(0.25x)^2既化为那种一个二次项的形式
所以S的最大值选B