x1 = a + b√2,x2 = c + d√2,则(1) x1 + x2 = (a+b) + (c+d)√2 ∈ A；x1x2 = (ac+2bd) + (ad+bc)√2 ∈ A.(2)x1/x2 = (a+b√2)/(c+d√2) = (a+b√2)(c-d√2)/(c²-2d²)= (ac-2bd)/(c²-2d²) + [(bc-...