Yahoo Answers: Answers and Comments for Did i do this correct? Look at this website. i dont know if i did it right? [Mathematics]
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From Anonymous
enUS
Tue, 26 Feb 2008 15:37:37 +0000
3
Yahoo Answers: Answers and Comments for Did i do this correct? Look at this website. i dont know if i did it right? [Mathematics]
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From gugliamo00: I'm going to presume those rounded ends ar...
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Wed, 27 Feb 2008 00:16:55 +0000
I'm going to presume those rounded ends are semicircles of diameter 70.
The Area (A) or the shaded rectantls will be the length (l) of one side multiplied the height (h). (A=lh)
We know the height to be 70 from our presumption.
Ok we also know the perimeter of the figure is 400
The circumference of the circular part (the two half circles) = πd
d = 70
c = 70π
For this one I’m going to use the 22/7 as the approximation for π.
70 x 22/7 = 220. Note: 22/7 isn't a very good approximation, but it makes this calculation easier.
The perimeter of the whole thing is 400. that includes the lengths of the two straight sides and the circumference of the circle. P = 400 = 2l+220
Add  220 to both sides
180 = 2l
Multiply both sides by 1/2
90=l
We're given h = 70
Substituting
A = lh
A = (90)(70) = 6300
Note: If you do this with a calculator, you're going to get a somewhat different answer because the calculator will use a closer approximation to π that 22/7.
For my own edification, I worked it out on a spreadsheet, and the approximation above is only about 3 square meters less than the calculation.

From DWRead: The two arcs make up 70π of the perimeter.
40...
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Tue, 26 Feb 2008 23:44:26 +0000
The two arcs make up 70π of the perimeter.
400  70π = 180m
the two outer sides are 180/2 = 90m each
the rectangle is 90m by 70m
area = 6300m²