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1. $a + b + c = 9, a^{2} + b^{2} + c^{2} = 29$

Answer: Option C

Explanation:

a + b + c = 9, a^{2} + b^{2} + c^{2} = 29 a b + b c + c a = ? {(a + b + c)}^{2} = a^{2} + b^{2} + c^{2} + 2 (a b + b c + c a) বা, ab+bc+ca = \{{(a + b + c)}^{2} - (a^{2} + b^{2})\} /2 বা, ab + bc + ca = \{{(a + b + c)}^{2} - (a^{2} + b^{2} + c^{2})\} / 2 বা, ab + bc + ca = \{{(a + b + c)}^{2} - (a^{2} + b^{2} + c^{2})\} / 2 = (81 - 29) / 2 = 26

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