Question 1111114
You use Heron's formula:
Area = sqrt(p(p-a)(p-b)(p-c)) where p is half the perimeter and a = 80, b = 90,and c = 100. So let's do it:
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(80+90+100)/2 = 135
Let's plug in all the numbers into the formula:
Area = sqrt(135(135-80)(135-90)(135-100))
= sqrt(135(55)(45)(35))
Now, because of the distributive properties of multiplication, we can either multiply 135 times each number in parenthesis and then multiply those numbers among themselves, or you can multiply the numbers in parenthesis and then multiply the result times 135, or just multiply across. This is what I am going to do: 
sqrt(135 x 55 x 45 x 35)
sqrt(11694375) = 3,419.70 m^2
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Happy learning,
John