a, \(\left|\overrightarrow{MA}+\overrightarrow{MB}+\overrightarrow{MC}\right|\)=\(\frac{3}{2}\)\(\left|\overrightarrow{MB}+\overrightarrow{MC}\right|\)
b, \(\left|\overrightarrow{MA}+\overrightarrow{BC}\right|\)=\(\left|\overrightarrow{MA}-\overrightarrow{MB}\right|\)