# how to find explicit solution

• Sep 19th 2009, 06:52 PM
lord12
how to find explicit solution
(1) Consider the initial value problem
dx/dt = -1/tx
, x(−1) = 1
(a) Use an analytic method to ﬁnd an explicit solution for this
problem. What is the domain of deﬁnition for this solution?

What does it mean to "use an analytic method"?
• Sep 19th 2009, 07:34 PM
Prove It
Quote:

Originally Posted by lord12
(1) Consider the initial value problem
dx/dt = -1/tx
, x(−1) = 1
(a) Use an analytic method to ﬁnd an explicit solution for this
problem. What is the domain of deﬁnition for this solution?

What does it mean to "use an analytic method"?

This is a separable DE

$\frac{dx}{dt} = -\frac{1}{tx}$

$x\,\frac{dx}{dt} = -\frac{1}{t}$

$\int{x\,\frac{dx}{dt}\,dt} = \int{-\frac{1}{t}\,dt}$

$\int{x\,dx} = -\ln{|t|} + C_1$

$\frac{1}{2}x^2 + C_2 = -\ln{|t|} + C_1$

$\frac{1}{2}x^2 = -\ln{|t|} + C_1 - C_2$

$x^2 = -2\ln{|t|} + 2C_1 - 2C_2$

$x^2 = -2\ln{|t|} + C$, where $C = 2C_1 - 2C_2$

$x = \sqrt{C - 2\ln |t|}$

Now use the initial condition to find C.
• Sep 20th 2009, 10:10 AM
lord12
but ln(-1) is undefined!
• Sep 20th 2009, 03:20 PM
mr fantastic
Quote:

Originally Posted by lord12
but ln(-1) is undefined!

No. It isn't. (For real numbers, anyway).

Luckily however, the solution you were given does not lead to this number .... (In other words, read the given answer more carefully).
• Sep 20th 2009, 07:36 PM
Prove It
Quote:

Originally Posted by lord12
but ln(-1) is undefined!

Does the answer ask you to find $\ln{(-1)}$?

No.

It asks you to find $\ln{|-1|} = \ln{1} = 0$.