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Question
If a + b + c = 9 and ab + bc + ca = 15, find: a2 + b2 + c2.
Sum
Solution
Since (a + b + c)2 = a2 + b2 + c2 + 2 (ab + bc + ca)
∴ (9)2 = a2 + b2 + c2 + 2 (15)
81 = a2 + b2 + c2 + 30
∴ a2 + b2 + c2 = 81 − 30 = 51
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Application of Formulae
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