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burhan101
 2 years ago
Solve for r
burhan101
 2 years ago
Solve for r

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burhan101
 2 years ago
Best ResponseYou've already chosen the best response.0\[\large 0=\frac{ r2700 }{ r^2 }+62 \pi r\]

calculusxy
 2 years ago
Best ResponseYou've already chosen the best response.0You are trying to find the area of a radius?

calculusxy
 2 years ago
Best ResponseYou've already chosen the best response.0Sorry I meant the area of a circle.

burhan101
 2 years ago
Best ResponseYou've already chosen the best response.0No it's an optimization problem I am trying to do, I just need to solve for 'r'

calculusxy
 2 years ago
Best ResponseYou've already chosen the best response.0Oh 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.

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

SmoothMath
 2 years ago
Best ResponseYou've already chosen the best response.1Oh my bad. You actually get a cubic.

burhan101
 2 years ago
Best ResponseYou've already chosen the best response.0top and bottom or just top?

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

burhan101
 2 years ago
Best ResponseYou've already chosen the best response.0\[r^2 or \frac{ r^2 }{ r^2 }\]

burhan101
 2 years ago
Best ResponseYou've already chosen the best response.0yeah so woulnt that cancel out with the fraction

SmoothMath
 2 years ago
Best ResponseYou've already chosen the best response.1Yes =) That's the point. It gets r out of the denominator.

burhan101
 2 years ago
Best ResponseYou've already chosen the best response.0ohh i thought you were also telling me to muktiply the top by r^3 so the function would be diff, i misunderstood

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

SmoothMath
 2 years ago
Best ResponseYou've already chosen the best response.1Gives: \(\huge 0 = r2700 + 62\pi*r^3\)

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

burhan101
 2 years ago
Best ResponseYou've already chosen the best response.0dw:1371520632051:dw yeah but the thing is like during an exam i cant use it :P

Bad2zBone
 2 years ago
Best ResponseYou've already chosen the best response.0Are you sure the question is correct?

Bad2zBone
 2 years ago
Best ResponseYou've already chosen the best response.0I suggest you double check, because you will not solve this without the aid of wolfaplha

burhan101
 2 years ago
Best ResponseYou've already chosen the best response.0here is the question and my solution

Bad2zBone
 2 years ago
Best ResponseYou've already chosen the best response.0I suggest you take a look at this http://gbbservices.com/math/cubic.html

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

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

SmoothMath
 2 years ago
Best ResponseYou've already chosen the best response.1Set 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?
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