SOLUTION: I need a formula to find r ( top radius) of a frustum given h 108cm,R(bottom radius)1.1m weight 8200Kg(density of concrete 2400Kg/m^3 I have been given V = 1/3 pi h (R^2+Rr+r^2

Algebra ->  Equations -> SOLUTION: I need a formula to find r ( top radius) of a frustum given h 108cm,R(bottom radius)1.1m weight 8200Kg(density of concrete 2400Kg/m^3 I have been given V = 1/3 pi h (R^2+Rr+r^2      Log On


   

Question 361937: I need a formula to find r ( top radius) of a frustum given h 108cm,R(bottom radius)1.1m weight 8200Kg(density of concrete 2400Kg/m^3
I have been given V = 1/3 pi h (R^2+Rr+r^2)
I am just so stuck !!!!!
Answer by Fombitz(32388) About Me  (Show Source):
You can put this solution on YOUR website!
Convert the weight to volume using the density, rho.
W=rho%2AV
V=W%2Frho=8200%2F2400=41%2F12m%5E3
.
.
.
V=%281%2F3%29%2Api%2Ah%2A%28R%5E2%2BRr%2Br%5E2%29=41%2F12
%281%2F3%29%2Api%2A0.108%2A%281.1%5E2%2B1.1r%2Br%5E2%29=41%2F12
r%5E2%2B1.1r%2B1.21=30.21
r%5E2%2B1.1r-30=0
Use the quadratic formula,
r+=+%28-1.1+%2B-+sqrt%28+1.1%5E2-4%2A1%2A%28-30%29+%29%29%2F%282%2A1%29+
You'll get two answers, one negative, one positive.
Only the positive one makes sense here.
The units for the answer will be meters.