$x\ \diamond\ y=(x+y)^2-(x-y)^2=x^2+2xy+y^2-(x^2-2xy+y^2)\\\\=x^2+2xy+y^2-x^2+2xy-y^2=4xy\\\\\sqrt5\ \diamond\ \sqrt5=4\cdot\sqrt5\cdot\sqrt5=4\cdot5=20$

Davood answered Aug 28, 2015

If x ¤ y = (x + y)² - (x - y)² Then √5 ¤ √5 =

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