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    Question

    If a2 + b2 + c2 = 21,

    (a + b + 7) = 21, then find the value of (ab + bc + ca).
    A 210 Correct Answer Incorrect Answer
    B 200 Correct Answer Incorrect Answer
    C 150 Correct Answer Incorrect Answer
    D 300 Correct Answer Incorrect Answer

    Solution

    We know, (a + b + c)2 = a2 + b2 + c2 + 2(ab + bc + ca) Or, (21)2 = 21 + 2(ab + bc + ca) Or, 441 = 21 + 2(ab + bc + ca) Or, ab + bc + ca = (441 – 21) ÷ 2 = 210

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