Tolong dijawab beserta caranya

[tex]18 + 8 \sqrt{5} = 2 {a}^{2} + 10 {b}^{2} + 4ab \sqrt{5} \\ 2 {a}^{2} + 10 {b}^{2} = 18 \\ 4ab \sqrt{5} = 8 \sqrt{5} \\ ab = 2 \\ b = \frac{2}{a} \\ 2 {a}^{2} + 10 {( \frac{2}{a} )}^{2} = 18 \\ 2 {a}^{4} + 40 = 18 {a}^{2} \\ 2{a}^{4} - 18 {a}^{2} + 40 = 0 \\ 2{( {a}^{2} )}^{2} - 18 {a}^{2} + 40 = 0 \\ ( {a}^{2} - 5)( {a}^{2} - 4) = 0 \\ {a}^{2} = 5 \: \: \: a = \sqrt{5} \: \: \: \: b = \frac{2}{ \sqrt{5} } \\ {a}^{2} = 4 \: \: \: a = 2 \: \: \: b = 1 \\ ba = 2[/tex]