Picture the top side of a circular medallion made of a circular piece of glass surrounded by a metal frame. If the radius of the medallion is r centimeters and the width of the metal frame is s centimeters, then in terms of s and r, what is the area of the metal frame, in square centimeters?
a) pi(r-s)^2
b) pi(r^2 - s^2)
c) 2pi(r-s)
d) pi*r(2r - s)
e) pi*s(2r - s)
I approached this problem two different ways and end up with the wrong answer both times. Could someone tell me where I'm going wrong?
Approach 1 --> Plug in values
Let r = 4
Let s = 1
Area of metal frame = Total Area - Area of glass medallion
Total area = Pi * (4 + 1)^2 = 25pi
Area of glass medallion = Pi * 4^2 = 16pi
Area of metal frame = 25pi - 16pi = 9pi
Plug my values into answer choice a and we get --> pi(4-1)^2 = 9pi, so this should be correct, but it isn't!
Approach 2 --> Algebra
Area of metal frame = Total Area - Area of glass medallion
Total area = Pi*(r + s)^2
Area of glass medallion = (Pi)(r^2)
Area of metal frame = (Pi)(r + s)^2 - (Pi)(r^2)
Pi(r^2 + 2rs + s^2) - Pi(r^2)
Pi(r^2) + 2Pi(rs) + Pi(s^2) - Pi(r^2)
2Pi(rs) + Pi(s^2)
Pi(s)(2r + s)
But this also isn't correct. What am I doing wrong?
a) pi(r-s)^2
b) pi(r^2 - s^2)
c) 2pi(r-s)
d) pi*r(2r - s)
e) pi*s(2r - s)
I approached this problem two different ways and end up with the wrong answer both times. Could someone tell me where I'm going wrong?
Approach 1 --> Plug in values
Let r = 4
Let s = 1
Area of metal frame = Total Area - Area of glass medallion
Total area = Pi * (4 + 1)^2 = 25pi
Area of glass medallion = Pi * 4^2 = 16pi
Area of metal frame = 25pi - 16pi = 9pi
Plug my values into answer choice a and we get --> pi(4-1)^2 = 9pi, so this should be correct, but it isn't!
Approach 2 --> Algebra
Area of metal frame = Total Area - Area of glass medallion
Total area = Pi*(r + s)^2
Area of glass medallion = (Pi)(r^2)
Area of metal frame = (Pi)(r + s)^2 - (Pi)(r^2)
Pi(r^2 + 2rs + s^2) - Pi(r^2)
Pi(r^2) + 2Pi(rs) + Pi(s^2) - Pi(r^2)
2Pi(rs) + Pi(s^2)
Pi(s)(2r + s)
But this also isn't correct. What am I doing wrong?

















