x ⋄ y = ( x + y ) 2 − ( x − y ) 2 = x 2 + 2 x y + y 2 − ( x 2 − 2 x y + y 2 ) = x 2 + 2 x y + y 2 − x 2 + 2 x y − y 2 = 4 x y 5 ⋄ 5 = 4 ⋅ 5 ⋅ 5 = 4 ⋅ 5 = 20
The calculation of 5 ¤ 5 using the defined operation results in 20. By applying the operation step-by-step, we found that ( 5 + 5 ) 2 equals 20 and ( 5 − 5 ) 2 equals 0. Therefore, the final answer is 20.
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