Given the first-order differential equation: d y </mrow>

pachaquis3s

pachaquis3s

Answered question

2022-06-24

Given the first-order differential equation:
d y d x = 6 x y
The textbook says you cannot differentiate both sides as y is on the right side and you have to use separation of variables. However, I did integrate both sides and arrived at this:
d y d x d x = 6 x y d x
y = 6 y x d x
y = 6 y ( x 2 2 + C )
y = 3 y x 2 + C
y + 3 y x 2 = C
y ( 1 + 3 x 2 ) = C
y ( x ) = C 1 + x 2
And the second last step is valid since 1 + 3 x 2 can never be zero. However, this is not the correct answer, which is:
y ( x ) = C e 3 x 2 . Why is this?

Answer & Explanation

lodosr

lodosr

Beginner2022-06-25Added 24 answers

Basically, you cannot take y off the integral, in the right part, since y=y(x). For example, if we have:
y ( x ) d x
If y ( x ) = e x , then
e x e x d x
e x e x x

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