18.
\(y'' + 4y = 3 \cos 2t\text{,}\) \(y(0) = 0\text{,}\) \(y'(0) = 0\)
19.
\(y'' + 7y' + 10 y = - 4 \sin 3t\text{,}\) \(y(0) = 0\text{,}\) \(y'(0) = 0\)
20.
\(\dfrac{d^2x}{dx^2} + 2 \dfrac{dx}{dt} + 2 x = 2 \cos 2t\text{,}\) \(x(0) = 0\text{,}\) \(x'(0) = 0\)
21.
\(x'' -2x' + 5x = 3 \cos t\text{,}\) \(x(0) = 0\text{,}\) \(x'(0) = 0\)
22.
\(y'' + 6y' + 7y= 3 \sin 2t\text{,}\) \(y(0) = 0\text{,}\) \(y'(0) = 0\)
23.
\(y'' + 4y' + 13y = 3 \cos 2t\text{,}\) \(y(0) = 0\text{,}\) \(y'(0) = 0\)
24.
\(y'' + 6y' + 8y = \cos 3t\text{,}\) \(y(0) = 0\text{,}\) \(y'(0) = 0\)