Solution
$$(a + b + c - d)^2 - (a - b - c + d)^2$$
[(a + b + c - d)+(a - b - c + d)][(a + b + c - d) - (a - b - c + d)]
($$\because a^2 - b^2 = (a + b)(a - b))$$
= (2a)(2b + 2c - 2d)
= 4a(b + c - d)
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