（x + y)^2 = x^2 + y^2 + 2xy = 26 + 2*3 = 32 (x - y)^2 = x^2 + y^2 - 2xy = 26 - 2*3 = 20
(x+y)²=ⅹ²+y²+2xy=26+2X3=32(ⅹ-y)²=ⅹ²+y²-2xy=26-2X3=20
