[tex]81 {x}^{2} + 16 = 0 \\ 81 {x}^{2} = - 16 | \div 81 \\ {x}^{2} = - \frac{16}{81} | \sqrt{} \\ x = \sqrt{ - \frac{16}{81} } \\ \\ x_1 = - \frac{4}{9} \\ x_2 = \frac{4}{9} [/tex]
