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First Order Differential Equation

  • Thread starter Woolyabyss
  • Start date
  • #1
143
1

Homework Statement


Find the general solution to the following differential equation.

dy/dx = 2x( (y^2) + 1)


Homework Equations





The Attempt at a Solution



I got all x terms on one side and all y terms on the otherside

2x dx = 1/( (y^2) + 1`)dy

integrate

x^2 + c = tan(y)

y = tan-inverse(x^2 + c)

Can somebody tell me if this is right ?

my book says the solution is y = tan(x^2 + c)
 

Answers and Replies

  • #2
phyzguy
Science Advisor
4,396
1,375
It's not right. What is
[tex] \int \frac{dy}{1+y^2}[/tex]?
 
  • #3
143
1
tan-inverse(1/y)

sorry I see where I went wrong now

it should have been x^2 + c = tan-inverse(y)
 
  • #4
1,540
134
tan-inverse(1/y)

sorry I see where I went wrong now

it should have been x^2 + c = tan-inverse(y)
∫dy/(1+y2) = tan-1y

So,tan-1y = x2 + c

or , y = tan(x2 + c)
 
  • #5
143
1
∫dy/(1+y2) = tan-1y

So,tan-1y = x2 + c

or , y = tan(x2 + c)
Could you help me with another one?

Given the differential equation

x(dy/dx) = √(4 - y^2) , find the general solution given that y = 0 when x = 1

I got all the x terms to one side and all of the y terms to the opposite side.

1/√(4 - y^2) dy = 1/x dx

integrating gives

sin-inverse(y/2) = lnx

using the limits and we get

sin-inverse(y/2) = lnx

y/2 = sin(lnx)

y = 2sin(lnx)

my book says the solution is y = sin(lnx)
 
  • #6
1,540
134
Could you help me with another one?

Given the differential equation

x(dy/dx) = √(4 - y^2) , find the general solution given that y = 0 when x = 1

I got all the x terms to one side and all of the y terms to the opposite side.

1/√(4 - y^2) dy = 1/x dx

integrating gives

sin-inverse(y/2) = lnx

using the limits and we get

sin-inverse(y/2) = lnx

y/2 = sin(lnx)

y = 2sin(lnx)

my book says the solution is y = sin(lnx)
y=2sin(lnx) looks to be the correct answer.You can verify that by putting the solution in the original equation and checking whether it satisfies the eq.

Note : Even though you came up with the correct answer ,please do not ignore the constant of integration C while integrating.
 

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