Tolong dibantu yaaa.. Makasihh

Kelas : IX SMP
Mapel : Matematika 
Bab : Bentuk Akar
Indikator : Operasi Bentuk Akar

Langkah Penyelesaian dan Jawaban :

[tex]a = \sqrt{7} + \sqrt{5} \\ b = \sqrt{7} - \sqrt{5} \\ (a + b) (a - b) = a^2-b^2 \\\\ \frac{ab}{a + b} = \frac{(\sqrt{7} + \sqrt{5})(\sqrt{7} - \sqrt{5})}{\sqrt{7} + \sqrt{5} + \sqrt{7} - \sqrt{5}} \\ \frac{ab}{a + b} = \frac{( \sqrt{7})^2 - ( \sqrt{5})^2}{2 \sqrt{7} } \\ \frac{ab}{a + b} = \frac{7 - 5}{2 \sqrt{7}} \\ \frac{ab}{a + b} = \frac{2}{2 \sqrt{7} } \\ \frac{ab}{a + b} = \frac{2}{2 \sqrt{7} } \ \cdot \frac{2 \sqrt{7}}{2 \sqrt{7}} \\ \frac{ab}{a + b} = \frac{4 \sqrt{7} }{28} \\[/tex]

[tex] \frac{ab}{a + b} = \frac{1}{7} \sqrt{7} \\ \therefore \boxed{\frac{1}{7} \sqrt{7}} [/tex]

[tex]a = \sqrt{7} + \sqrt{5} \\ b = \sqrt{7} - \sqrt{5} \\ \frac{ab}{a + b} = \frac{ (\sqrt{7} + \sqrt{5})( \sqrt{7} - \sqrt{5}) }{( \sqrt{7} + \sqrt{5}) + ( \sqrt{7} - \sqrt{5}) } \\ = \frac{7 - \sqrt{35} + \sqrt{35} - 5 }{2 \sqrt{7} } \\ = \frac{2}{2 \sqrt{7} } \\ = \frac{2}{2 \sqrt{7} } \times \frac{2 \sqrt{7} }{2 \sqrt{7} } \\ = \frac{4 \sqrt{7} }{4 \times 7} \\ = \frac{4 \sqrt{7} }{28} \\ = \frac{ \sqrt{7} }{7} [/tex]
semoga bermanfaat dan maaf jika salah