Question 224624
Your FIRST question is actually "What is the area of a circle with a 40 foot radius?" Many people can casually quote "Pie are square", (well actually its πR^2), but most don't know that it's actually the equation to calculate the area of a circle.  Pi (the mathematical symbol is π) is an endless "never repeating" number that is roughly 3.14159, "R" is the radius of a circle or arc and 2 (or "squared") is any number times itself.

Whenever we have an "unknown" in a equation we use "X" as a placeholder for it.  So, using the formula above, we would state our question mathematically by plugging in the radius to say:   X = 3.14159 * 40^2
  
If your calculator has a couple of "special" buttons on it, this will be REALLY easy.  One is the Pi symbol (remember, it looks like this: π).  The other may have something like ^2 printed on it.  That means "raised to the power of 2", or "squared".

If you DO have these buttons, you can enter your equation like this:

    π * 40^2 =

and get your result.

If you only have a basic calculator and DON'T have the special buttons, you can do it like this:

    3.14159 * 40 * 40

and get the same thing.


The SECOND question involves subtracting the area of one geometric figure (the circle) from another one (the square).  Since you know the radius of the circle (40 feet), you know that the side of the square is equal to the diameter of the circle (or TWICE the radius).  The formula for the area of a square is simply "the length of the side squared", or S^2.

    Since the side of the square is 80 (two times the radius), you enter:

    80^2

to get the area of the square.  Now, if you subtract the area of the circle (from the first problem) from the area of the square, the only thing LEFT is the corners!  But be careful!  The answer to your FIRST question was in square feet, but the second question asked for an answer in ACRES!  

No problem.  One acre is equal to 43560 square feet, so if we divide our answer (above) by 43560, that will give us our final answer in acres.


Now, I was pretty careful to tell you HOW to do it, without telling you what the answer actually WAS.  If you paid attention, you should have no problem plugging in the numbers for the answer.  Good luck.


"Give a man a fish, feed him for a day.  TEACH a man to fish, feed him for a lifetime!"

RMc