# Math Help - Separation of variables

1. ## Separation of variables

Solve the separable differential equation y'(x) = sqrt(2y(x)+15)
and find the particular solution satisfying the initial condition y(1) = -3

Solve the separable differential equation y'(x) = sqrt(2y(x)+15)
and find the particular solution satisfying the initial condition y(1) = -3
$y' = \sqrt{2y+15}$

$\frac{y'}{\sqrt{2y+15}} = 1$

$\int \frac{y'dx}{\sqrt{2y+15}} = \int 1 dx$

Continue.

3. Thanks, I first thought the x is actually y times x instead of the function of y with variable x.

Well, I got y = (x^2 - 7)/2

Sounds right?

Thanks, I first thought the x is actually y times x instead of the function of y with variable x.

Well, I got y = (x^2 - 7)/2

Sounds right?
Is $y^{\prime} = \sqrt{2y + 15}$?

Let me give you a hand:
$\int \frac{y}{\sqrt{2y + 15}}~dy$

Let $u = 2y + 15 \implies du = 2 dy$

$\int \frac{y}{\sqrt{2y + 15}}~dy = \int \frac{u - 15}{2} \cdot \frac{1}{\sqrt{u}} \cdot \frac{dy}{2}$

$= \frac{1}{4} \int \left ( \sqrt{u} -
\frac{15}{\sqrt{u}} \right ) du$

Can you take it from here?

-Dan

5. Yeah, looks like I made a silly mistake up there.

Now, I get to a point where I found C = {2,-4}.

So y = [(x+2)^2 - 15]/2 or y = [(x-4)^2 - 15]/2

Are they both the right answers?

Yeah, looks like I made a silly mistake up there.

Now, I get to a point where I found C = {2,-4}.

So y = [(x+2)^2 - 15]/2 or y = [(x-4)^2 - 15]/2

Are they both the right answers?
Originally Posted by topsquark
Is $y^{\prime} = \sqrt{2y + 15}$?

Let me give you a hand:
$\int \frac{y}{\sqrt{2y + 15}}~dy$

Let $u = 2y + 15 \implies du = 2 dy$

$\int \frac{y}{\sqrt{2y + 15}}~dy = \int \frac{u - 15}{2} \cdot \frac{1}{\sqrt{u}} \cdot \frac{dy}{2}$

$= \frac{1}{4} \int \left ( \sqrt{u} -
\frac{15}{\sqrt{u}} \right ) du$

Can you take it from here?

-Dan
Let me continue.
$= \frac{1}{4} \int \left ( \sqrt{u} -
\frac{15}{\sqrt{u}} \right ) du$

$= \frac{1}{4} \left ( \frac{2}{3}u^{3/2} + 2 \cdot 15u^{1/2} \right ) + C$

$= \frac{1}{6}(2y + 15)^{3/2} + \frac{15}{2}(2y + 15)^{1/2} + C$

So we have the differential equation:

$y^{\prime} = \sqrt{2y + 15}$

$\frac{dy}{\sqrt{2y + 15}} = dx$

$\int \frac{dy}{\sqrt{2y + 15}} = \int dx$

The LHS is just the integral I finished up at the top, so
$\frac{1}{6}(2y + 15)^{3/2} + \frac{15}{2}(2y + 15)^{1/2} = x + C$

And we know $y(1) = -3$.

Thus
$\frac{1}{6}(2 \cdot -3 + 15)^{3/2} + \frac{15}{2}(2 \cdot -3 + 15)^{1/2} = 1 + C$

$C = \frac{1}{6}(9)^{3/2} + \frac{15}{2}(9)^{1/2} - 1 = \frac{27}{6} + \frac{45}{2} - 1 = 26$

So finally:
$\frac{1}{6}(2y + 15)^{3/2} + \frac{15}{2}(2y + 15)^{1/2} = x + 26$

Now to solve for y.
$\frac{1}{6}(2y + 15)^{3/2} = -\frac{15}{2}(2y + 15)^{1/2} + x + 26$ <-- Square both sides:

$\frac{1}{36}(2y + 15)^3 = \left ( -\frac{15}{2}(2y + 15)^{1/2} + x + 26 \right )^2$

$\frac{1}{36}(2y + 15)^3 = x^2 - (15x + 390)\sqrt{2y + 15} + 52x + \frac{225y}{2} + \frac{6079}{4}$

Now solve this for $\sqrt{2y + 15}$, square both sides again, solve the polynomial for y, and check the solutions to make sure there are no extra ones in there. It'll keep you busy for a while, to say the least.

-Dan