a) Ta có:
\(\eqalign{
& {x^2} - 7 = {x^2} - {\left( {\sqrt 7 } \right)^2} \cr
& = \left( {x + \sqrt 7 } \right)\left( {x - \sqrt 7 } \right) \cr} \)
b) Ta có:
\(\eqalign{
& {x^2} - 2\sqrt 2 x + 2 \cr
& = {x^2} - 2.x.\sqrt 2 + {\left( {\sqrt 2 } \right)^2} \cr
& = {\left( {x - \sqrt 2 } \right)^2} \cr} \)
c) Ta có:
\(\eqalign{
& {x^2} + 2\sqrt {13} x + 13 \cr
& = {x^2} + 2.x.\sqrt {13} + {\left( {\sqrt {13} } \right)^2} \cr
& = {\left( {x + \sqrt {13} } \right)^2} \cr} \)
