一题数学 看不太明白

thomasyao

The figure shows the top side of a circular medallion made of a circular piece of colored glass surrounded by a medal frame, represented by the shaded region (一个环形的东东). If the radius of the medallion is r cm and the width of medal frame is s cm. then in terms of s & r, what's the area of the medal frame, in squre cm?

B. PI *(r^2-s^2)

E. PI * s(2*r-s)

Why E, not C?

谢谢！

michaelwzg

a circular medallion / radius =r      (奖章半径r)

a medal frame/ surround medallion / width =s (外面套一个框宽度是s)

the area of the medal frame (外面那个框的面积)

=整个面积-奖章面积=PI*(S+R)^2-PI*R^2=E

thomasyao

谢谢，原来我对于框宽度的理解出错了。