Integrating a separable differential equation in differential form. I am studying differential equations from a book called Differential Equations (Schaum's outlines). In it, it says if we have a separable differential equation: A(x) dx +B(y) dy=0, y(x_0)=y_0

Winston Todd

Winston Todd

Answered question

2022-10-19

Integrating a separable differential equation in differential form
I am studying differential equations from a book called Differential Equations (Schaum's outlines). In it, it says if we have a separable differential equation:
A ( x ) d x + B ( y ) d y = 0 ,   y ( x 0 ) = y 0
then, the solution to the initial-value problem can be obtained by
x 0 x A ( x ) d x + y 0 y B ( y ) d y = 0
And that's my problem. I was expecting something like
x 0 x A ( x ) d x + x 0 x B ( y ) d y = 0
as we integrate the first equation from x o to x
I admit that what i am stating is not so intuitive (in fact, the second equation makes more sense than the third). But how can i prove that
x 0 x B ( y ( x ) )   d y ( x ) = y 0 y B ( y ) d y

Answer & Explanation

dkmc4175fl

dkmc4175fl

Beginner2022-10-20Added 15 answers

I think it's easier to think about if we rewrite the differential equation as:
A ( x ) + B ( y ) d y d x = 0
Now integrate both sides with respect to x (with limits x 0 and x) to give:
x 0 x A ( x ) d x + x 0 x B ( y ( x ) ) d y d x d x = 0
In the second integral, make the change of variable y = y ( x ) (i.e. integration by substitution) and apply your initial condition, so that:
x 0 x B ( y ( x ) ) d y d x d x = y ( x 0 ) y ( x ) B ( y ) d y = y 0 y B ( y ) d y
... as stated!
In your final line, the expression
x 0 x B ( y ( x ) )   d y ( x )
doesn't really make sense. The symbols " d x" and " d y" have a number of different interpretations. Both of the statements d y = d y d x d x and d y = y ( x ) d x make sense, but d y ( x ) is dodgy.
Nigro6f

Nigro6f

Beginner2022-10-21Added 2 answers

Taking the derivative on x of
x 0 x A ( x ) d x + x 0 x B ( y ) d y = C
you obtain
A ( x ) + B ( x ) = 0 ,
which is certainly not the original ODE.
On the opposite
x 0 x A ( x ) d x + y 0 y ( x ) B ( y ) d y
gives
A ( x ) + B ( y ( x ) ) d y ( x ) d x .

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