Each graph below represents a solution to one of the following differential equations. Match the graph to the differential equation.

(a) $x'' + x = 0$ matches graph ? 1 2 3 4
(b) $x'' + 4 x = 0$ matches graph ? 1 2 3 4
(c) $x'' + 16 x = 0$ matches graph ? 1 2 3 4

(d) Find an equation for each of the graphs:
For graph (1), $x(t) =$
For graph (2), $x(t) =$
For graph (3), $x(t) =$
For graph (4), $x(t) =$