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2013-11-29T10:48:43-02:00
(9a-b) . (9a+b)
81a²+9ab-9ab-b²
81a²-b²
letra c..
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2013-11-29T10:49:07-02:00
(9a-b).(9a+b)=81a^{2}+9ab-9ab-b=81a^{2}-b{2}
9a.9a=81a^{2}
9a.b=9ab
-b.9a=-9ab
-b.b=-b^{2}