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خطا

${{v}_{av}}=\frac{\Delta x}{\Delta t}=\frac{\Delta {{x}_{1}}+\Delta {{x}_{2}}}{\Delta {{t}_{1}}+\Delta {{t}_{2}}}=\frac{300+(-900)}{\frac{\Delta {{x}_{1}}}{{{v}_{1}}}+\frac{\Delta {{x}_{2}}}{{{v}_{2}}}}=\frac{300-900}{\frac{300}{15}+\frac{-900}{-30}}=\frac{-600}{20+30}=-12\frac{m}{s}\Rightarrow \left| {{v}_{av}} \right|=12\frac{m}{s}$