\(\overrightarrow{AB}+\overrightarrow{AD}=\overrightarrow{AC}\). Nên \(\left|\overrightarrow{AB}+\overrightarrow{AD}\right|=\left|\overrightarrow{AC}\right|\)\(=AC\). Áp dụng định lý Pi-ta-go: \(AC=\sqrt{AB^2+BC^2}=\sqrt{\left(4a\right)^2+\left(3a\right)^2}=5a\).