Khẳng định nào sau đây sai?

\(\int {\left[ {f\left( x \right) + g\left( x \right)} \right]} dx = \int {f\left( x \right)dx} + \int {g\left( x \right)dx} \);

\(\int {\left[ {f\left( x \right) - g\left( x \right)} \right]} dx = \int {f\left( x \right)dx} - \int {g\left( x \right)dx} \);

\(\int {kf\left( x \right)dx} = k\int {f\left( x \right)dx} ,\forall k \in \mathbb{R}\);

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