This program uses a 3rd Order Runge-Kutta method to assist in solving a first order-differential equation.

Given the initial condition (x0, y0) to the differential equation:

dy/dx = f(x, y)

Find the value of y1 where h is a given step size (preferably small). The value of y1 is given by the following equations:

y1 = y0 + k1/4 + (3 * k3)/4

Where:

k1 = h * f(x0, y0)

k2 = h * f(x0 + h/3, y0 + k1/3)

k3 = h * f(x0 + (2 * h)/3, y0 + (2 * k2)/3)

Error estimated on the order of h^4

Source: Smith, Jon M. Scientific Analysis on the Pocket Calculator. John Wiley & Sons, Inc.: New York 1975 pg 174

**Memory Registers Used**

R0 = h

R1 = x

R2 = y

R3 = x1 (result)

R4 = y1 (result)

R5 = x0 (copied from R1)

R6 = y0 (copied from R2)

R7 = k1

R8 = k2

R9 = k3**Labels Used**

Label A = Main Program

Label 0 = Program where equation is maintained**Instructions**

1. Store h, x0, and y0 in memory registers R0, R1, and R2 respectively.

2. Enter or edit the equation in program mode using Label 0. Use RCL 1 for x and RCL 2 for y.

3. Run Program A. The first answer is x1. Press [R/S] for y1.

4. To find (x2, y2), store x1 in R1 and y1 in R2 and run Program A again. You can do this by pressing [STO] [ 2 ] [x<>y] [STO] [ 1 ] [ f ] [ √ ] (A) immediately following program execution.

There are 59 program steps in the main program.

Key Code

LBL A 42 21 11

RCL 1 45 1

STO 5 44 5

RCL+ 0 45 40 0

STO 3 44 3

RCL 2 45 2

STO 6 44 6

GSB 0 32 0

RCLx 0 45 20 0

STO 7 44 7

RCL 5 45 5

RCL 0 45 0

3 3

÷ 10

+ 40

STO 1 44 1

RCL 6 45 6

RCL 7 45 7

3 3

÷ 10

+ 40

STO 2 44 2

GSB 0 32 0

RCLx 0 45 44 0

STO 8 44 8

RCL 5 45 5

RCL 0 45 0

2 2

x 20

3 3

÷ 10

+ 40

STO 1 44 1

RCL 6 45 6

RCL 8 45 8

2 2

x 20

3 3

÷ 10

+ 40

STO 2 44 2

GSB 0 32 0

RCLx 0 45 20 0

STO 9 44 9

RCL 3 45 3

R/S 31

RCL 6 45 6

RCL 7 45 7

4 4

÷ 10

+ 40

RCL 9 45 9

3 3

x 20

4 4

÷ 10

+ 40

STO 4 44 4

RTN 43 32

Example 1

dy/dx = x^2 + sin(x * y)

Initial Conditions: (1, 1), let h = 0.01

Program for equation:

LBL 0

RAD

RCL 1

x^2

RCL 1

RCLx 2

SIN

+

RTN

Results:

x y

1.0000 1.0000

1.0100 1.0186

1.0200 1.0375

1.0300 1.0568

Example 2

dy/dx = x^3 * y - y

Initial Condition: (1,1); Step size h = 0.01

Program for the equation:

LBL 0

RCL 1

3

y^x

RCLx 2

RCL- 2

RTN

Results:

x y

1.0000 1.0000

1.0100 1.0002

1.0200 1.0006

1.0300 1.0014

*This blog is property of Edward Shore. © 2012*

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