Discussion about math, puzzles, games and fun.   Useful symbols: ÷ × ½ √ ∞ ≠ ≤ ≥ ≈ ⇒ ± ∈ Δ θ ∴ ∑ ∫ • π ƒ -¹ ² ³ °

You are not logged in.

## Post a reply

Write your message and submit
|
Options

## Topic review (newest first)

anonimnystefy
2012-10-09 18:52:31

I also think that in 2, you calculated the function incorrectly in the second step.

Eileen
2012-10-09 12:02:35

Thankyou very much to both of you!

bob bundy
2012-10-09 02:22:46

hi Eileen

Q1.

That looks good to me, but I'm very confused by what you then did.

It's a linear differential equation requiring an 'integrating factor'.

Re-write as

The integrating factor is just (t + 25)

t= 0 A = 0 makes C = 0

Put t = 25

So concentration is

Bob

anonimnystefy
2012-10-08 19:20:26

Hi

For 2, you should first find the constant c from the two initial conditions given, and only then find the flow rate.

Eileen
2012-10-08 10:58:37

I am having difficulty getting the correct answers to two mixing problems. If anyone would kindly help me out, it'd be much appreciated!

Problem1:
A tank initially holds 25 gal of water. Salt enters at a rate of 2gal/min and the mixture leaves at a rate of 1 gal/min. What will be the concentration of salt when 50 gal of fluid is in the tank?
dA/dt=2-[A(t)/(t+25)]
IF=(t+25)
A(t+25)t^2)+50t+c
A=[(t^2)+50t+c]/(t+25)
For A(0)=0; c=0
A=[(t^2)+50t]/(t+25)
50=[(t^2)+50t]/(t+25)
t=35.36
35.36/50=.71

The answer should be 75%.

Problem 2:
A tank initially contains 100 gal of a solution that holds 30lb of a chemical. A solution containing 2 lb/gal of the chemical runs in at a constant rate and the mixture runs out at the same rate. What should be the rate of flow if the tank is to contain 70lb of the chemical after 40min?
dA/dt=2r-(A(t)*r/100)
A(t)=ce^(-rt/100)+200
70=ce^(-40r/100)+200
-130=ce^(-2r/5)
ln(-130)=ln(c)*ln(e^-2r/5)
ln(-130)=c*2r/5
(5/2)ln(-130)=r

The answer should be (5/2)ln(17/13)=.671

## Board footer

Powered by FluxBB