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    • Question

      If (x + y + z) = 10 and (x 2  + y 2

       + z 2 ) = 52, then find {z(x + y) + xy}.
      A 36 Correct Answer Incorrect Answer
      B 32 Correct Answer Incorrect Answer
      C 48 Correct Answer Incorrect Answer
      D 24 Correct Answer Incorrect Answer

      Solution

      Since, (x + y + z) = x 2  + y 2  + z 2  + 2(xy +yz +zx)

      So, (10) 2   = 52 + 2(xy +yz +zx)

      Or, 100 - 52 = 2(xy +yz +zx)

      Or, 48 = 2(xy +yz +zx)

      Or, 24 = (xy +yz +zx)

      So, z(x + y) + xy = 24

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