Try: circle area, U = I × R, radius
Euler, Heun, Runge–Kutta and Newton iteration steps.
y₁ = y₀ + hf(x₀,y₀)
y₁ = y₀ + h(k₁+k₂)/2
y₁ = y₀ + h(k₁+2k₂+2k₃+k₄)/6
x₁ = x₀ − f(x₀)/f′(x₀)