Separable Differential Equations

Solve separable differential equations for AP Calculus AB.

Separable differential equations can be solved by separating variables. This is the main DE-solving technique for AP Calculus AB.

Method

dydx=f(x)g(y)\frac{dy}{dx} = f(x)g(y)

  1. Separate: 1g(y)dy=f(x)dx\frac{1}{g(y)}\,dy = f(x)\,dx.
  2. Integrate both sides.
  3. Solve for yy (if possible).
  4. Use initial condition to find CC.

Worked Example: Example 1

Problem

dydx=2xy\frac{dy}{dx} = 2xy, y(0)=3y(0) = 3.

dyy=2xdx\frac{dy}{y} = 2x\,dxlny=x2+C\ln|y| = x^2 + Cy=Aex2y = Ae^{x^2}.

y(0)=3y(0) = 3: A=3A = 3. Solution: y=3ex2y = 3e^{x^2}.

Solution

Worked Example: Example 2

Problem

dydx=x+1y2\frac{dy}{dx} = \frac{x+1}{y^2}, y(0)=1y(0) = 1.

y2dy=(x+1)dxy^2\,dy = (x+1)\,dxy33=x22+x+C\frac{y^3}{3} = \frac{x^2}{2} + x + C.

y(0)=1y(0) = 1: 13=C\frac{1}{3} = C. So y3=3x22+3x+1y^3 = \frac{3x^2}{2} + 3x + 1.

Solution

Practice Problems

    1. dydx=yx\frac{dy}{dx} = \frac{y}{x}, y(1)=4y(1) = 4.
    1. dydx=ycosx\frac{dy}{dx} = y\cos x, y(0)=2y(0) = 2.

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Key Takeaways

  • Separate all yy terms with dydy, all xx terms with dxdx.

  • Integrate both sides.

  • Apply initial conditions to find CC.

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