Solve DE: Find Solution for y Through (-1,-1)

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SUMMARY

The discussion focuses on solving the differential equation \(\frac{dy}{dt}=y\cos(t)\) with the initial condition passing through the point (-1, -1). The solution involves integrating both sides, leading to \(\ln|y| = \sin(t) + C\). The key insight is recognizing that \(\ln(y)\) should be treated as \(\ln|y|\) to accommodate negative values, allowing for the solution \(y = D \cdot e^{\sin(t)}\), where \(D\) can be negative to satisfy the initial condition.

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Homework Statement



[tex]\frac{dy}{dt}=y\cos(t)[/tex]

Find the solution of the DE that passes through the point (-1, -1).

Homework Equations


The Attempt at a Solution



[tex]\frac{dy}{dt}=y\cos(t)[/tex]

[tex]\frac{1}{y}dy=cos(t)dt[/tex]

integrate both sides:

[tex]ln(y) = sin(t) + C[/tex]

Normally I would plug in -1 for y and t and solve for C but I can't take the LN of -1. When I try to isolate y first then plug in, I get the same problem with t. How can I solve this? I am stuck! Is it that the ln(y) is actually ln(|y|) ?
 
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You figured it out for yourself in the end:smile::

[tex]\int\frac{dy}{y}=\ln|y|[/tex]
 
Last edited:
You could do the absolute value, since ln(-y)'=ln(y)=1/y*y'. Or you could just exponentiate your solution to get y=exp(sin(t)+C)=D*exp(sin(t)). Now you can put D negative.
 

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