2nd Order ODE and the ti89

Numerically:
Single ode of 2nd order can be convert to a system of two 1st order
odes. (Hint: y1(t) is the function and y2(t) is y1'(t), so
		y'' + y' + y = 0 becomes (let y2 be y' and y1 be y)
		y1'(t) = y2(t) and y2'(t) = - y2(t) - y1(t).

Analytically: (deSolve is F3 C)
	deSolve(y''+4y=0,t,y)
		yields @1 cos(2t) + @2 sin(2t)
		(@1 = C1 an arbitrary constant, @2 = C2 another arbitrary constant)
	deSolve(y''+4y=0 and y(0)=1 and y'(0)=0,t,y)
		yields cos(2t)

	The ti-89 will also solve non-homogenuous equations but not by the
	method of undetermined coefficients. Instead it seems to use variation
	of parameters.