Representing differential equations explicitly and implicitly For the past two weeks I have been dealing with differential equations and I have stumbled upon a seemingly simple problem. Suppose I have a separable differential equation (dy)/(dx)=y(x). This simply tells that the function value has to equal to the slope at a particular x value. The solution is therefore simply e^x . So far so good. Now suppose I have similiar equation, namely (dy)/(dx)=x^2 x^2 . Now this tells me that the slope of the function at a particular x has to equal x^2. Hence, the solution is simply the integral, namely 1/3x^3. One does not even need the conventional methods to solve separable differential equations. However, let me now define y(x)=x^2. The two differential equations (and hence the solutions) should

Martin Hart

Martin Hart

Answered question

2022-10-27

Representing differential equations explicitly and implicitly
For the past two weeks I have been dealing with differential equations and I have stumbled upon a seemingly simple problem. Suppose I have a separable differential equation d y d x = y ( x ). This simply tells that the function value has to equal to the slope at a particular x value. The solution is therefore simply e x . So far so good. Now suppose I have similiar equation, namely d y d x = x 2 . Now this tells me that the slope of the function at a particular x has to equal x 2 . Hence, the solution is simply the integral, namely 1 3 x 3 . One does not even need the conventional methods to solve separable differential equations.
However, let me now define y ( x ) = x 2 . The two differential equations (and hence the solutions) should be now equivalent. As we have seen, that's not the case. I can sense that I am commiting a mathematical fallacy, however, I do not know what seems to be the problem when I solve the differential equation implicitly rather than explicitly as stated above. Could anyone help me please?

Answer & Explanation

Krystal Dillon

Krystal Dillon

Beginner2022-10-28Added 10 answers

You found that a solution to d y d x = x 2 is y ( x ) = 1 3 x 3 , so now when you set y ( x ) = x 2 , you change y ( x ) from the previous y ( x ) = 1 3 x 3 and it is no longer a solution to the equation. The new y ( x ) is a solution to d y d x = 2 x, because d d x ( x 2 ) = 2 x. The equation d y d x = y ( x ) is only satisfied by a function that is equal to its derivative everywhere and neither 1 3 x 3 nor x 2 are such functions. Family of functions y ( x ) = C e x , C R , are only solutions.

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