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The sum of the squares of three numbers is 138, while the sum of their products taken two at a time is 131. Their sum is:

The sum of the squares of three numbers is 138, while the sum of their products taken two at a time is 131. Their sum is: Correct Answer 20

Let the numbers be a, b and c
Then, a2 + b2 + c2 = 138 and (ab + bc + ca) = 131
(a + b + c)2 = a2 + b2 + c2 + 2(ab + bc + ca) = 138 + 2 x 131 = 400
⇒ (a + b + c) = $$\sqrt {400} $$  = 20

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