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= 9 (a-b-c)(a-b-c)
= 9 [ a (a-b-c) - b (a-b-c) - c (a-b-c) ]
= 9 [ a²-ab-ac - ab+b²+bc-ac+bc+c² ]
= 9 [ a²+b²+c² - 2ab-2ac+2bc ]