a) \(\left| {\overrightarrow {AB} + \overrightarrow {AD} } \right| = \left| {\overrightarrow {AC} } \right|;\), a) \(\left| {\overrightarrow {AB} + \overrightarrow {AD} } \right| = \left| {\overrightarrow {AC} } \right|\);