# Thread: problem solving area equation using pi

1. ## problem solving area equation using pi

Hi all,

I am having some trouble solving a basic area equation. I have the values and the answer but am unable to recreate it myself. This is a problem as I need to apply the same equation to solve my own values (smaller 2's indicate squared, I couldn't find the symbol for this, nor for pi):

Area (km2) = pi(D/d)2

Where D= 2.5, d=0.5. The answer I have for this is 4.9 km2.

The equation I need to solve is for:

pi(14/7)2

Any help would be appreciated. I have tried a combination of things like pi x (D/d
2), the squared inside and outside the bracket but am getting nowhere...

E

2. ## Re: problem solving area equation using pi

Originally Posted by glxej
Hi all,

I am having some trouble solving a basic area equation. I have the values and the answer but am unable to recreate it myself. This is a problem as I need to apply the same equation to solve my own values (smaller 2's indicate squared, I couldn't find the symbol for this, nor for pi):

Area (km2) = pi(D/d)2

Where D= 2.5, d=0.5. The answer I have for this is 4.9 km2.

The equation I need to solve is for:

pi(14/7)2

Any help would be appreciated. I have tried a combination of things like pi x (D/d
2), the squared inside and outside the bracket but am getting nowhere...

E

3. ## Re: problem solving area equation using pi

Originally Posted by glxej
Hi all,

I am having some trouble solving a basic area equation. I have the values and the answer but am unable to recreate it myself. This is a problem as I need to apply the same equation to solve my own values (smaller 2's indicate squared, I couldn't find the symbol for this, nor for pi):

Area (km2) = pi(D/d)2

Where D= 2.5, d=0.5. The answer I have for this is 4.9 km2.

The equation I need to solve is for:

pi(14/7)2

Any help would be appreciated. I have tried a combination of things like pi x (D/d
2), the squared inside and outside the bracket but am getting nowhere...

E
$\displaystyle \pi \left(\dfrac{14}{7}\right)^2 = 4\pi$

Which is just a number.

4. ## Re: problem solving area equation using pi

Sorry, not quite sure what is meant by the 'entire problem'. The above is all I have to work with....unless you mean the units for D and d, which are both in km.

5. ## Re: problem solving area equation using pi

Originally Posted by glxej
The equation I need to solve is for:
pi(14/7)2
That is NOT an equation; all equations have an equal sign.
Most of us here like to help, but NOT to "guess"...get my drift?

If you're trying to evaluate that expression, then:
pi times (14/7) to the power 2
= 3.141... times 2 to the power 2
= 3.141... times 4
= 12.564...

On this one:
> Area (km2) = pi(D/d)2
> Where D= 2.5, d=0.5. The answer I have for this is 4.9 km2.
3.141... * (2.5/.5)^2 = 3.141... * 5^2 = 3.141... * 25 = 78.525...
Your 4.9 makes no sense whatsoever...

WHAT are you trying to do? WHERE does that "formula" come from?
WHAT are D and d ? Are you working with circles (area pi * r^2) ?

6. ## Re: problem solving area equation using pi

Alright, so I am trying to work out the Topographic Area (A) (km2) knowing two diameters (D) and (d) both in km.

The formula is from an academic scientific journal, I am trying to use their values to recreate their answer, and so that I may apply it to my own problem. However, I am unable to get the same answer which poses problems as the value is used later on in other formulas,

The formula is:
Area (km2) = pi(D/d)2

Their application:
Area (km2) = pi(2.5/0.5)2 = 4.9 km2

Or another:
Area (km2) = pi(9/5)2 = 64 km2

I would like to apply it to:
Area (km2) = pi(14/7)2 = ? km2

Again, I cannot get the answer they get!? Any ideas how to solve this?

(ps. Strangely, the next equation I am needed to was to work out the Structure Area (Sa). They use pretty much the same formula;
Sa (km2) = pi(d/2) (again d=km)
so for example, Sa (km2) = pi*(0.5/2)=~0.2km2 ....I am able to get the answer myself and it matches their values, but why not for the first formula!!)

7. ## Re: problem solving area equation using pi

Here's roughly what you're saying:

3 * 5 = 15

"they" have 3 * 5 = 60

any idea how "they" got that?

8. ## Re: problem solving area equation using pi

Originally Posted by glxej
Alright, so I am trying to work out the Topographic Area (A) (km2) knowing two diameters (D) and (d) both in km.

The formula is from an academic scientific journal, I am trying to use their values to recreate their answer, and so that I may apply it to my own problem. However, I am unable to get the same answer which poses problems as the value is used later on in other formulas,

The formula is:
Area (km2) = pi(D/d)2

Their application:
Area (km2) = pi(2.5/0.5)2 = 4.9 km2
No, 2.5/.5= 5, 5^2= 25 and 25pi= 25(3.14)= 78.5

Or another:
Area (km2) = pi(9/5)2 = 64 km2
Again, no. 9/5= 1.8. 1.8^2= 3.24, and pi(3.24)= 10.17

However, the whole problem makes no sense if R and r are radii, measured in km, the "R/r" has no units. multiplying by pi will NOT give an area in "square kilometers".

I would like to apply it to:
Area (km2) = pi(14/7)2 = ? km2

Again, I cannot get the answer they get!? Any ideas how to solve this?

(ps. Strangely, the next equation I am needed to was to work out the Structure Area (Sa). They use pretty much the same formula;
Sa (km2) = pi(d/2) (again d=km)
so for example, Sa (km2) = pi*(0.5/2)=~0.2km2 ....I am able to get the answer myself and it matches their values, but why not for the first formula!!)

9. ## Re: problem solving area equation using pi

Originally Posted by glxej
pi*(0.5/2)=~0.2km2 ....I am able to get the answer myself and it matches their values,...
Huh? 3.14 * (.05/2) = 3.14 * .025 = ~.0785
Are you joking around?