Câu 9. Khẳng định nào sau đây sai?

\(\int {\left[ {f\left( x \right) + g\left( x \right)} \right]{\rm{d}}x} = \int {f\left( x \right){\rm{d}}x} + \int {g\left( x \right){\rm{d}}x} .\)

\(\int {\left[ {f\left( x \right) - g\left( x \right)} \right]{\rm{d}}x} = \int {f\left( x \right){\rm{d}}x} - \int {g\left( x \right){\rm{d}}x} .\)

\(\int {kf\left( x \right){\rm{d}}x} = k\int {f\left( x \right){\rm{d}}x} ,\forall k \in \mathbb{R}.\)

\(\int {kf\left( x \right){\rm{d}}x} = k\int {f\left( x \right){\rm{d}}x} ,\forall k \in \mathbb{R},k \ne 0.\)