dx/dt = f (x, y)
dy/dt = g (x, y)
with initial conditions x(0) = x_0 and y(0) = y_0,
Euler's algorithm with step size h becomes
x_(n+1) = x_n + h*f(x_n, y_n)
y_(n+1) = y_n + h*g(x_n, y_n).
Now consider the Epidemics problem from last week. Take a = 0.6, r = 0.003, S_0 = 200, I_0 = 20, and step size h = 0.2. Compute two steps of Euler by hand. Compare your numbers with those from Phaser.