Anonymous
Anonymous asked in Science & MathematicsMathematics · 1 decade ago

Did i do this correct? Look at this website. i dont know if i did it right?

http://www.aasd.k12.wi.us/East/academics/math/Alle...

look at #5

my work:

i used the equation A= .5aP

note: a is apothem. P is Perimeter

.5(70)(400)= 14,000

2 Answers

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  • 1 decade ago
    Favorite Answer

    I'm going to presume those rounded ends are semi-circles 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.

  • DWRead
    Lv 7
    1 decade ago

    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

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