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#1 2010-04-22 18:40:13
need conformation on a math formula
in geometry we just learned volume and surface area of sphere's, though I believe I learned volume in 7th grade however surface area I didn't, ok I just need you to check these formulas and tell me mine works...
textbooks formula: 4pi x radius squared
my formula: circumference x diameter [or] diameter squared x pi
I believe mine is wrong because I'm wondering if it should be: Cir x Cir/2
basically it means the area of there circumference for the length of half the circumference, though the other one is the area of the circumference for the length of the diameter, which doesn't make any sense at all to me because diameter does not cover the half the actual length of a sphere (Cir/2)
I also found another formula for volume related to my formula on SA, though mine is more complex...
textbooks formula: 4/3pi x radius cubed (cubed is the third power)
my formula: diameter x (pi radius squared)
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#2 2010-04-22 18:45:13
Re: need conformation on a math formula
For the 4/3pi x radius cubed, the textbook is correct. For the other, also textbook is correct. And your formula should be correct if you add Cir x Cir/2.
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#3 2010-04-22 18:48:23
Re: need conformation on a math formula
well yes, but for some reason when I got the answer for a problem using the basic formula and then used mine (Cir x Diam) both answers were the same, I performed this on several other different problems and the answers were still the same, though I'll make up a few and test all 3
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#4 2010-04-22 18:55:18
Re: need conformation on a math formula
diameter=7
text=153.9380400258998686846695257807
CxD=153.9380400258998686846695257807
CxC/2=241.80530782668928616144502949697 (I also tried this with a calculator that I could write out the problem and solve it one step, but it still came out the same)
I have no clue at all why this is happening...
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#5 2010-04-22 19:25:23
Re: need conformation on a math formula
Surface area(SA):
4pi * r^2
Volume(V):
4/3pi * r^3
2r = d
SA:
4pi * d/2^2
4pi * d^2/4
pi * d^2
V:
4/3pi * d/2^3
4/3pi* d^3/8
1/3pi* d^3/2
(pi* d^3)/(3*2)
pi*d^3/6
I'm pretty sure from here you can work down.
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