3) 400x² − 16 = 0

\(400x^2=16\).

\(x^2=\frac{16}{400}=\frac{1}{25}\).

\(x=\pm\frac{1}{5}\).

Ответ: \(x_1=\frac{1}{5}\), \(x_2=-\frac{1}{5}\).