SOLUTION: Find the complete or particular solution on the following differential equations: y'-2y=y^2 ; y=3 when x=0 Answer: 5y=3(y+2)e^(2x)

Algebra ->  Expressions -> SOLUTION: Find the complete or particular solution on the following differential equations: y'-2y=y^2 ; y=3 when x=0 Answer: 5y=3(y+2)e^(2x)      Log On


   



Question 1196935: Find the complete or particular solution on the following differential equations:
y'-2y=y^2 ; y=3 when x=0
Answer: 5y=3(y+2)e^(2x)

Answer by Edwin McCravy(20060) About Me  (Show Source):
You can put this solution on YOUR website!

%22y%27%22-2y=y%5E2

This is a Bernoulli differential equation with n = 2, the exponent of y

let u = y1-n = y1-2 = y-1, then

%22u%27%22=-y%5E%28-2%29%2A%22y%27%22

We can make the first term of the original equation become u' by 
multiplying through by -y%5E%28-2%29

-y%5E%28-2%29%2A%22y%27%22-%28-y%5E%28-2%29%292y=%28-y%5E%28-2%29%29y%5E2

-y%5E%28-2%29%2A%22y%27%22%2B2y%5E%28-1%29=-y%5E0

-y%5E%28-2%29%2A%22y%27%22%2B2y%5E%28-1%29=-1

Now we also see that since y-1 = u, the above becomes

%22u%27%22%2B2u=-1

du%2Fdx%2B2u=-1

du%2B2udx=-dx

That is separable as well as a linear differential equation. I will
solve it as a linear differential equation, with P = 2 and Q = -1.
so we find the integrating factor 

matrix%282%2C1%2C%22%22%2Ce%5Eint%28P%2Adx%29%29matrix%282%2C1%2C%22%22%2C%22%22=%22%22%29matrix%282%2C1%2C%22%22%2Ce%5Eint%282%2Adx%29%29matrix%282%2C1%2C%22%22%2C%22%22=%22%22%29matrix%282%2C1%2C%22%22%2Ce%5E%282x%29%29 
  
Multiplying through by e2x, 

e%5E%282x%29du%2B2u%2Ae%5E%282x%29dx=-e%5E%282x%29%2Adx

The left side is the differential of the product (e2x)(u),
so we can integrate both terms on the left together.  

int%28%28e%5E%282x%29du%2B2u%2Ae%5E%282x%29dx%29%29%22%22=%22%22int%28-e%5E%282x%29%2Adx%29

On the right we take out the negative sign, insert 2 and take out 1/2:

e%5E%282x%29%2A%28u%29%22%22=%22%22expr%28-1%2F2%29int%28e%5E%282x%29%2A2%2Adx%29

e%5E%282x%29%2A%28u%29%22%22=%22%22expr%28-1%2F2%29e%5E%282x%29%22%22%2B%22%22C

We are told y=3 when x=0

u = y-1, substituting y=3

u = 3-1 = 1%2F3

e%5E%282%2A0%29%2A%281%2F3%29%22%22=%22%22expr%28-1%2F2%29e%5E%282%2A0%29%22%22%2B%22%22C

1%2A%281%2F3%29%22%22=%22%22expr%28-1%2F2%29%2A1%22%22%2B%22%22C

1%2F3%22%22=%22%22-1%2F2%22%22%2B%22%22C

1%2F3%2B1%2F2%22%22=%22%22C

5%2F6%22%22=%22%22C

e%5E%282x%29%2A%28u%29%22%22=%22%22expr%28-1%2F2%29e%5E%282x%29%22%22%2B%22%225%2F6

Since  u = y-1,  

e%5E%282x%29%2A%28y%5E%28-1%29%29%22%22=%22%22expr%28-1%2F2%29e%5E%282x%29%22%22%2B%22%225%2F6

Multiply through by 6

6e%5E%282x%29%2A%28y%5E%28-1%29%29%22%22=%22%22-3e%5E%282x%29%2B5

6e%5E%282x%29%2A%281%2Fy%29%22%22=%22%22-3e%5E%282x%29%2B5

Multiply through by y

6e%5E%282x%29%22%22=%22%22-3ye%5E%282x%29%2B5y


3ye%5E%282x%29%2B6e%5E%282x%29%22%22=%22%225y

Factor out 3e2x on the left

3e%5E%282x%29%28y%2B2%29%22%22=%22%225y

Swap sides:

5y%22%22=%22%223e%5E%282x%29%28y%2B2%29

5y%22%22=%22%223%28y%2B2%29e%5E%282x%29

Edwin