Câu 1
a) (√18 + 1/2 * 12√2) / √2 = (3√2 + 6√2 ) / √2 = 9√2 / √2 = 9
b)$\left \{ {{3x-y=15} \atop {x+y=-1}} \right.$
⇔$\left \{ {{3x-y+x+y=15-1} \atop {x+y=-1}} \right.$
⇔$\left \{ {{4x=14} \atop {x+y=-1}} \right.$
⇔$\left \{ {{x=\frac{7}{2}} \atop {\frac{7}{2}+y=-1}} \right.$
⇔$\left \{ {{x=\frac{7}{2}} \atop {y=\frac{-9}{2}}} \right.$
Vậy...