Question
If x + y + z = 9 and (x2 + y2 +
z2) = 21, then find the value of (xy + yz + zx)Solution
(x + y + z)2 = x2 + y2 + z2 + 2 × (xy + yz + xz) Or, 92 = 21 + 2 × (xy + yz + xz) Or, 2 × (xy + yz + xz) = 81 – 21 Or, (xy + yz + xz) = 60 ÷ 2 = 30
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