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have you done radians?

|dw:1433038449394:dw|
does this make sense?

so what would be the area of the full circle i've drawn?

\[area= \pi*r ^{2}\]

yes

now we want to find the sector with angle 120

not quite.

full circle = 360 degrees

if we divide 360 by 120 we get three

so that means 120 is 1/3 of 360. right?

Right!!

now for the next part...

|dw:1433039404347:dw|

\[area of full circle = \pi * r ^{2}\] ... but this time r =5

yes!

now for the area of the sector of angle 120.

area of full circle (360 degrees) = 78.53... so area of sector of angle 120 would be 1/3 of 78.53...

yh we're caculating the white part of raius 5

*radius

the second diagram is only showing the white part

@alexwaldorf do you under stand the calculation?

yeah, sorry about interrupting, my mistake D:

so what would be the area of the sector in the second part? @alexwaldorf

not quite. area of full circle = 78.53982.. area of sector of angle 120 would be 1/3 of that.

yes!... do you understand why it's 1/3 of full area?

26*

good. finally we have...

|dw:1433040636347:dw|

area of sector of radius 20 (15+5) = 418.87.. and area of sector of radius 5 (white part) = 23.17...

does that make sense?

yes it does

so is 23.17 the answer?

not quite. the area we need to find is the black part.

|dw:1433041243267:dw|

that is correct but if you put all the decimals in from both calculations you will get 392.6990817.

yes

yes

no problem. you're welcome

I've got to go now...bye