What is the area of the largest rectangle with lower base on the x-axis and upper vertices on the curve y = 3 − x2?

Answer:
(C) 4
Step-by-step explanation:
Let us first construct a rectangle ABCD, in which A is the point at lower right hand corner and B,C,D are the points marked according to A,
Now, let A= [tex](p,0)[/tex]
B=[tex](-p,0)[/tex],
C=[tex](-p,3-p^{2})[/tex]
and D=[tex](p,3-p^{2})[/tex]
Then the area of rectangle is given as: BA×AD
=[tex](2p)(3-p^{2})[/tex]
A=[tex]6p-2p^{3}[/tex]
Taking the derivative with respect to p, we have
[tex]A^{'}[/tex]=[tex]6-6p^{2}[/tex]
Now, [tex]A^{'}[/tex]=0
⇒[tex]6-6p^{2}[/tex]=0
⇒[tex]6p^{2}=6[/tex]
⇒[tex]p^{2}=1[/tex]
Since, wehav eto find the greater area, therefore we will take p=1.
Now, substituting the value of p in (A), we have
Greater area= A=[tex]6-2(1)[/tex]=[tex]4 sq units[/tex]
Answer:
C. 4
Step-by-step explanation:
After plotting the curves, we get the figure given below.
So, the rectangle will lie in 1st and 2nd quadrant.
Thus, let the vertex in the 1st quadrant = ( x,y ) and in 2nd quadrant = ( -x,y ).
Then, the length of the rectangle = 2x and width of the rectangle = y.
As, area of a rectangle = length × width
Therefore, area of the given rectangle, A = 2x × y
i.e. [tex]A=2x(3-x^{2})[/tex]
i.e. [tex]A=6x-2x^{3}[/tex]
Thus, differentiating with respect to x and equating to 0 gives,
[tex]\frac{dA}{dx}=0[/tex]
i.e. [tex]6-6x^{2}=0[/tex]
i.e. [tex]6x^{2}=6[/tex]
i.e. [tex]x^{2}=1[/tex]
i.e. [tex]x=1,-1[/tex]
Again, differentiating with respect to x gives us [tex]\frac{d^{2}A}{dx^{2}} =-12x[/tex].
If x = -1 , [tex]\frac{d^{2}A}{dx^{2}} =12>0[/tex].
If x =1 , [tex]\frac{d^{2}A}{dx^{2}} =-12<0[/tex]. This gives us that the maximum value of the area is obtained at x = 1.
Thus, length = 2x = 2 and width = y = [tex]3-x^{2}[/tex] = 3- 1 = 2
So, area of the rectangle is A = 2(1) × 2 = 4.
Hence, area of the rectangle is 4 [tex]unit^{2}[/tex].