# DSolve returns true

Posted 4 years ago
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 I keep checking videos and online tutorials via youtube and Mathematica website however, I still can't get a simple DSolve equation to work. Why is this giving me true? I am trying to Solve it so I can plot solutions and slope fields. Equation is Dy/Dx = x-y In: Clear[x, y] In: DSolve[y'[x] == x - y[x], y[x], x] Out: DSolve[True, y[x], x] 
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Posted 4 years ago
 Using Mathematica 7, I got a solution (* Commands 1*) Clear[x, y]; DSolve[ y'[x] == x - y[x], y[x], x] {{y[x] -> -1 + x + E^-x C[1]}} The only time I was able to reproduce the "DSolve[True...." message was when I had the sequence of commands as follows, Clear[x, y]; y'[x] = x - y[x]; DSolve[y'[x] == x - y[x], y[x], x] What's happening is that y'[x] is known to be x- y[x], and the DSolve is doing a compare of y-y[x] == y-y[x], returning True. Then it can't solve True for y[x] in x, resulting in the question being returned.Would you try exiting the kernel, restart fresh, and re-typing (* commands 1*) only.
 Values of parameters (and functions and anything else you define in a Mathematica session) are independent of the window that you are in (i.e, they're defined in a Kernel session, not a window generally--there's an exception to this that is subtle). If you want to clear out all the definitions and value assignments you have made in a Mathematica session then execute this ClearAll["Global*"] and if you want to remove them completely from the Mathematica session then execute Remove["Global*"]