使用平方差公式 a^2 - b^2 = (a + b)(a - b) x ¤y = (x + y)^2 - (x - y)^2 = (x + y + x - y)(x + y - x + y) = (2x)(2y) = 4xy √5 ¤√5 = 4(√5)(√5) = 4(5) = 20 因此，答案为选项 E