Posted by nabiha on Friday, April 23, 2010 at 5:55pm.
a cylindrical cake with radius 12cm and height 10cm has a slice cut out. The shape of the top of the slice is a sector of the circle that forms the top of the cake. Exluding the sliced piece, the angle is 320 degrees.
a) Calculate the area on top of the slice that has been cut out.
b) Calculate the volume of the cake that remains after the slice has been removed.
c) Calculate the surface area of the cake that remains after the slice has been removed.
i particularly need to know how to do c) This is because I have already worked out the answers to a) and b)
c)
I worked out the area of the cake using the sector 320 degrees, but there is also a bottom so you therefore have to multiple it by 2.
same with when you cut the slice there are 2 rectangular sides. obviously round the circumference of the cake there is a long rectangular shape but because it is a cake it looks curved. because you want the whole surface area once you've sliced it you have to add all the figures. Here are my workings:
10*12=120*2=240cm squared
area of base=320 over 360*pie*12 squared=402.1238597*2 cos theres a top and bottom 4 a cake= 804.2477193
w=320 over 360*2*pie*12 ( multiply this by height of 10cm= 670.2064328
add these: 240+804.2477193+670.2064328=1714.454852 so surface area = 1414cm squared
however in the book the answer was 1474cm squared. i figured out they didn't add the 240 on which are the bits on the side which is right?

maths  Reiny, Sunday, April 25, 2010 at 8:58am
It's me again.
I now fully understand what you are asking for in your part c)
I agree with your calculations of the exposed area of the top and bottom of the remaining cake
area (top+bottom) = 2(320/360)(144π) = 804.25 (you had that)
area (exposed outer side) = (320/360)(2π(12))(10) = 670.21 (you had that also)
if we add just those two sums we get 1474.45 which is their answer.
So it does look like they forgot the 2 rectangular areas of the inside of the remaining cake, which you calculated correctly as well, which were 2(12)(10) = 240
so the correct answer should have been the sum of those 3 answers or 1714.45 which you also had.
I again apologize for my error in our last exchange.

maths  nabiha, Sunday, April 25, 2010 at 1:16pm
thankyou, I wanted to know whether I was right or the textbook because it was very rare that they make mistakes, I wanted to clarify whether it makes sense to add the 2 sides area=240. I thought it did so Im going to leave it how I did it.
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