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burhan101Best ResponseYou've already chosen the best response.0
\[\large 0=\frac{ r2700 }{ r^2 }+62 \pi r\]
 10 months ago

calculusxyBest ResponseYou've already chosen the best response.0
You are trying to find the area of a radius?
 10 months ago

calculusxyBest ResponseYou've already chosen the best response.0
Sorry I meant the area of a circle.
 10 months ago

burhan101Best ResponseYou've already chosen the best response.0
No it's an optimization problem I am trying to do, I just need to solve for 'r'
 10 months ago

calculusxyBest ResponseYou've already chosen the best response.0
Oh I thought that because the formula to finding the area of a circle,you have to first square the radius and then multiply it by pi (r^2)3.14.
 10 months ago

SmoothMathBest ResponseYou've already chosen the best response.1
Multiply everything by r^2. Then factor out an r. From there it's a quadratic.
 10 months ago

SmoothMathBest ResponseYou've already chosen the best response.1
Oh my bad. You actually get a cubic.
 10 months ago

burhan101Best ResponseYou've already chosen the best response.0
top and bottom or just top?
 10 months ago

SmoothMathBest ResponseYou've already chosen the best response.1
Multiply everything by r^2. When you multiply the fraction, multiply on top.
 10 months ago

burhan101Best ResponseYou've already chosen the best response.0
\[r^2 or \frac{ r^2 }{ r^2 }\]
 10 months ago

burhan101Best ResponseYou've already chosen the best response.0
yeah so woulnt that cancel out with the fraction
 10 months ago

SmoothMathBest ResponseYou've already chosen the best response.1
Yes =) That's the point. It gets r out of the denominator.
 10 months ago

burhan101Best ResponseYou've already chosen the best response.0
ohh i thought you were also telling me to muktiply the top by r^3 so the function would be diff, i misunderstood
 10 months ago

SmoothMathBest ResponseYou've already chosen the best response.1
\(\huge 0 = \frac{r2700}{r^2} + 62\pi*r\) multiply everything by r^2 \(\huge (0)*r^2 = (\frac{r2700}{r^2})*r^2 + (62\pi*r)*r^2\)
 10 months ago

SmoothMathBest ResponseYou've already chosen the best response.1
Gives: \(\huge 0 = r2700 + 62\pi*r^3\)
 10 months ago

SmoothMathBest ResponseYou've already chosen the best response.1
And honestly from there your best bet is to use a cubic solver of some sort. Wolfram Alpha should do just fine.
 10 months ago

burhan101Best ResponseYou've already chosen the best response.0
dw:1371520632051:dw yeah but the thing is like during an exam i cant use it :P
 10 months ago

Bad2zBoneBest ResponseYou've already chosen the best response.0
Are you sure the question is correct?
 10 months ago

Bad2zBoneBest ResponseYou've already chosen the best response.0
I suggest you double check, because you will not solve this without the aid of wolfaplha
 10 months ago

burhan101Best ResponseYou've already chosen the best response.0
here is the question and my solution
 10 months ago

Bad2zBoneBest ResponseYou've already chosen the best response.0
I suggest you take a look at this http://gbbservices.com/math/cubic.html
 10 months ago

SmoothMathBest ResponseYou've already chosen the best response.1
Okay on the step where you take C', you make incorrect use of the quotient rule. The derivative of the top will be 0.
 10 months ago

SmoothMathBest ResponseYou've already chosen the best response.1
The correct derivative is: \(\Large C' = \frac{27900}{r^2} + 62\pi*r\)
 10 months ago

SmoothMathBest ResponseYou've already chosen the best response.1
Set the derivative equal to 0 and solve: \(\Large \frac{27900}{r^2} + 62\pi*r = 0\) \(\Large (\frac{27900}{r^2} + 62\pi*r)*r^2 = 0*r^2\) \(\Large 27900 + 62\pi*r^3 = 0\) \(\Large 62\pi*r^3 = 27900\) You good from there?
 10 months ago
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