(a + b) . (x + y) =

= a . (x + y) + b . (x + y )

= ax + ay + bx + by

–

(a + b) . (a – b) =

= a . (a – b) + b . (a – b)

= aa – ab + ba – bb

= a2 - ab + ab – b2

= a2 – b2

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