Question

a square + b square + C is equal to 20 and a + b + C is equal to zero find a b + BC + CA

Answer 1

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${( a + b + c )}^{2} = \\ {a }^{2} + {b }^{2} + {c}^{2} + 2(ab + bc + ac) \\ = > {0}^{2} = 20 + \: 2(ab + bc + ac) \\ = > - 20 = 2(ab+bc+ac) \\ = > - 10 = ab+bc+ac \:$