Nếu và thì bằng

* Hướng dẫn giải

Ta có \(\int\limits_1^2 {\left[ {2f\left( x \right) + g\left( x \right)} \right]{\rm{d}}x} = 13 \Leftrightarrow 2.\int\limits_1^2 {f\left( x \right){\rm{d}}x} + \int\limits_1^2 {g\left( x \right){\rm{d}}x} = 13\)

\( \Leftrightarrow \int\limits_1^2 {g\left( x \right){\rm{d}}x} = 13 - 2.\int\limits_1^2 {f\left( x \right){\rm{d}}x} \Leftrightarrow \int\limits_1^2 {g\left( x \right){\rm{d}}x} = 13 - 2.5 \Leftrightarrow \int\limits_1^2 {g\left( x \right){\rm{d}}x} = 3\).

Vậy \(\int\limits_1^2 {g\left( x \right){\rm{d}}x} = 3\).