正方形的面积为“ d”。以正方形的对角线为直径的圆的面积是多少?
(A) πd
(B)πD2
(C)(1/4)πD2
(D) (1/2)πd答案： (D)
解释： 
One important observation to solve
the question :
Diagonal of Square = Diameter of Circle.
Let side of square be x.
From Pythogorous theorem.
Diagonal = √(2*x*x)
We know area of square = x * x = d
Diameter = Diagonal = √(2*d)
Radius = √(d/2)
Area of Circle = π * √(d/2) * √(d/2) = 1/2 * π * d
这个问题的测验