Solution:
Let the numbers be a, b and c.
Then, a2 + b2 + c2 = 138 and (ab + bc + ca) = 131.
WKT, (a + b + c) 2 = a2 + b2 + c2 + 2(ab + bc + ca)
Substitutinga2 + b2 + c2 = 138,(ab + bc + ca) = 131.
(a + b + c)2= 138 + (2 x 131)
(a + b + c)2= 138 + 262
(a + b + c)2= 400
(a + b + c) = 20.