a2 – b2 = (a – b)(a + b).
Примеры.
x2 – 4 = (x – 2)(x + 2)
1 – 9a2 = (1 – 3a)(1 + 3a)
69 ⋅ 71 = (70 – 1)(70 + 1) = 702 – 1 = 4900 – 1 = 4899.
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