Cho hàm số $y=f\left(x\right)$ liên tục trên đoạn $\left[ a;b\right]$. Mệnh đề nào dưới đây sai?

	$\displaystyle\displaystyle\int\limits_{a}^{b}f\left(x\right)\mathrm{d}x=\displaystyle\displaystyle\int\limits_{a}^{b}f\left(t\right)\mathrm{d}t$
	$\displaystyle\displaystyle\int\limits_{a}^{b}{f\left(x\right)\mathrm{d}x}=-\displaystyle\displaystyle\int\limits_{b}^{a}{f\left(x\right)\mathrm{d}x}$
	$\displaystyle\displaystyle\int\limits_{a}^{b}k\mathrm{d}x=k\left(a-b\right)$, $\forall k\in\mathbb{R}$
	$\displaystyle\displaystyle\int\limits_{a}^{b}f\left(x\right)\mathrm{d}x=\displaystyle\displaystyle\int\limits_{a}^{c}f\left(x\right)\mathrm{d}x+\displaystyle\displaystyle\int\limits_{c}^{b}f\left(x\right)\mathrm{d}x$