Tính nhanh

$$\begin{gathered} \mathrm{Tính} \mathrm{nhanh}: \\ 723+\left[246+\left(-223\right)+254\right] \\ 912+\left[88+\left(-453\right)+\left(-547\right)\right] \\ \mathrm{Tính} \mathrm{tổng}: \\ \mathrm{S}=(-1)+2+\left(-3\right)+4\left(-5\right)+6...+\left(-2013\right)+2014 \\ \mathrm{Thực} \mathrm{hiện} \mathrm{các} \mathrm{phép} \mathrm{tính} \mathrm{sau}: \\ 6-8 \\ 3-\left(-9\right) \\ \left(-5\right)-10 \\ 0-7 \\ 4-0 \\ \left(-2\right)-\left(-10\right) \\ \mathrm{Tìm} \mathrm{x}\in \mathrm{Z} \mathrm{trong} \mathrm{mỗi} \mathrm{trường} \mathrm{hợp} \mathrm{sau}: \\ \mathrm{x}+3=20 \\ \mathrm{x}+12=4 \\ \mathrm{x}+15=0 \\ \mathrm{x}-5=7 \\ 6-\mathrm{x}=8 \\ -\mathrm{x}-4=10 \end{gathered}$$