- #1
Northbysouth
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Homework Statement
A tank initially contains 180 gallons of water in which 8grams of salt are dissolved.
Water containing 9 grams of salt per gallon enters the tank at the rate of 3 gallons
per minute, and
the well mixed solution leaves the tank at the rate of 1 gallon per minute. The equation for the
amount of salt in the tank for anytime t.
Homework Equations
The Attempt at a Solution
My confusion is with the integrating factor.
First off I know:
Q' = rate at which salt enters - rate at which salt leaves
Q' = (9 grams/gal)(3 gal/min) - Q/(180 + 2t)
Simplify to Differential equation format
Q' + Q/(180 + 2t) = 27
Find the integrating factor.
u = e∫1/(180 + 2t) dt = e1/2 ln(180 + 2t)
My question is how do you know to move the 1/2 into the power to give the integrating factor u as:
u = (180 + 2t)1/2
I know it's a logarithmic law that I can move the coefficient into the power, but how do I recognize that I need to do this here? What's wrong with leaving the half in front as:
u = 1/2(180 + 2t)
Guidance would be appreciated. Thank you.