In Problems 19 through 22, first derive a recurrence relation giving c_n for n ≧ 2 in terms of c₀ or c₁ (or both). Then apply the given initial conditions to find the values of c₀ and c₁. Next determine c_n (in terms of n, as in the text) and, finally, identify the particular solution in terms of familiar elementary functions.

y'' + 4y = 0; y(0) = 0, y'(0) = 3