Double factorial joke

the-joke

Recipe:
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let x be a positive integer
let a be: 2x!! - 2x
let b be: 2x!! - 4x

For a most tasteful joke, let x be 5, 6, or 7.

Contemplate the reality that the following two statements are true:
 a - b  / 2 = x!!
(a - b) / 2 = x

... and the more uncanny one that the double factorial operator is eerily reminiscent of a rather enthusiastic punctuating mark
... and allow yourself to overlook that it is here denoted as two separate bangs.

The joke is to claim, quite excitedly, that:
a - b / 2 = x!!

Examples:
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20 - 10 / 2 = 5!!
84 - 72 / 2 = 6!!
196 - 182 / 2 = 7!!