How can I solve this non-linear differential equation? y'=1-y^{2}

Kaspaueru2

Kaspaueru2

Answered question

2022-01-16

How can I solve this non-linear differential equation?
y=1y2

Answer & Explanation

xandir307dc

xandir307dc

Beginner2022-01-17Added 35 answers

y=1y2
Divide by (1y2)
y1y2=1
Integrate both sides:
12log|y+1y1|=t+c
Rearrange y=ke2t+1ke2t1
I'd have thought that solution was right, but we have to figure out a specific solution with y(0)=0. But this isn't possible with the above equation.
peterpan7117i

peterpan7117i

Beginner2022-01-18Added 39 answers

Since you want a solution near y=0, you should use 1y in the denominator (as it will be positive) and can remove the absolute value signs. This changes some signs in your answer, giving y=ke2t1ke2t+1
and k=1 gives y(0)=0
alenahelenash

alenahelenash

Expert2022-01-24Added 556 answers

The first thing you should notice is that y=1 and y=1 are the two constant solutions, which allows you then to divide y' by 1y2, since you want to study it for y(0)(1,1), knowing that any solution starting in (1,1) stays there (or dies in 1).Then with some algebra you manage to get(log1+y1y)=2if I haven't screwed up with the signs; now integrating it from 0 to t you get:log1+y(t)1y(t)log1+y(0)1y(0)=2twithout the absolute value since everything in the argument of the logs is non-negative. By imposing y(0)=0 the second term in the left vanishes and you're left with an easy expression that if inverted gives the following:y(t)=e2t1e2t+1which is simplyy(t)=tanh(t)and of course double checking y(0)=0.

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