a/ \(E_{1M}=\frac{kq_1}{AM^2};E_{2M}=\frac{kq_2}{BM^2}\)
\(\Rightarrow\sum E=E_{1M}+E_{2M}=\frac{9.10^9.4.10^{-6}}{0,4^2}+\frac{9.10^9.9.10^{-6}}{0,9^2}=...\left(V/m\right)\)
b/ \(\sum E=\left|E'_{1M}-E'_{2M}\right|=|\frac{9.10^9.4.10^{-6}}{0,25^2}-\frac{9.10^9.9.10^{-6}}{0,25^2}|=...\left(V/m\right)\)