3 1/a 1/b 1/c

Lời giải:

Ta có:

abc = a + b + c

\(\frac{{abc}}{{abc}} = \frac{{a + b + c}}{{abc}}\)

\(1 = \frac{1}{{ab}} + \frac{1}{{bc}} + \frac{1}{{ca}} = Q\)

Ta có: \({\left( {\frac{1}{a} + \frac{1}{b} + \frac{1}{c}} \right)^2} = \frac{1}{{{a^2}}} + \frac{1}{{{b^2}}} + \frac{1}{{{c^2}}} + 2\left( {\frac{1}{{ab}} + \frac{1}{{bc}} + \frac{1}{{ca}}} \right)\)

Suy ra \(P = {3^2} - 2Q = 9 - 2 = 7\)