Obtaining Differential Equations from Functions\frac{dy}{dx}=x^{2}-1is a first orde

Linda Seales

Linda Seales

Answered question

2022-01-18

Obtaining Differential Equations from Functions
dydx=x21
is a first order ODE,
d2ydx2+2(dydx)2+y=0
is a second order ODE and so on. I am having trouble to obtain a differential equation from a given function. I could find the differential equation for
y=ex(Acosx+Bsinx) and the steps that I followed are as follows.
dydx=ex(Acosx+Bsinx)+ex(Asinx+Bcosx)
=y+ex(Asinx+Bcosx) (1)
d2ydx2=dydx+ex(Asinx+Bcosx)+ex(AcosxBsinx)
=dydx+(dydxy)y
using the orginal function and (1). Finally,
d2ydx22dydx+2y=0,
which is the required differential equation.
Similarly, if the function is y=(Acos2t+Bsin2t), the differential equation that I get is
d2ydx2+4y=0
following similar steps as above.

Answer & Explanation

reinosodairyshm

reinosodairyshm

Beginner2022-01-19Added 36 answers

An answer has already been given for y=Ae3x+Be2x, so we deal with
xy=Aex+Bex+x2 (Equation 1)
Differentiate both sides of the given equation twice with respect to x. We first get
xdydx+y=AexBex+2x, (Equation 2)
and then, differentiating again,
xd2ydx2+2dydx=Aex+Bex+2 (Equation 3)
Now look at Equation 1 and Equation 3. The term Aex+Bex occurs in each. Subtract, and it disappears. We obtain
xd2ydx2+2dydxxy=2x2
Shawn Kim

Shawn Kim

Beginner2022-01-20Added 25 answers

Generally, in such problems, if the function involves k arbitrary constants, we expect to differentiate k times. Then, we should eliminate the arbitrary constants and obtain a differential equation in terms of
y,dydx,d2ydx2,,dkydxk.
We will see how this works out in the case of
y=Ae3x+Be2x (1)
Since there are two arbitrary constants A and B, we expect to involve y and its first two derivatives. Differentiating twice, we get
dydx=3Ae3x+2Be2x, (2)
and d2ydx2=9Ae3x+4Be2x. (3)
Now, how do we eliminate A and B from these equations? We can eliminate B from (2) and (3) by subtracting a suitable multiple of (1). That is, (2)2×(1) gives:
dydx2y=Ae3x. (4)
Similarly, (3)4×(1) gives:
d2ydx24y=5Ae3x. (5)
To get the differential equation, we now need to similarly eliminate A from these equations (4) and (5). Do you see how?
alenahelenash

alenahelenash

Expert2022-01-24Added 556 answers

Eliminate a and b from y=ae2x+be3x (1) dydx=2ae2x+3be3x (2) d2y/dx2=4ae2x+9be3x (3) (1)2(2) we get 2ydydx=be3x b=e3x(dydx2y) (4) (2)2(3) 2dydxd2ydx2=3be3x b=13e3x(d2ydx22dydx) (5) compare (4) and (5) (dydx2y)=13(d2ydx22dydx) 3dydx6y=d2ydx22dydx d2ydx25dydx+6y=0

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