Let ABC be a triangle,\(\overrightarrow{BC}=\overrightarrow{a},\overrightarrow{CA}=\overrightarrow{b}\) and \(\overrightarrow{AB}=\overrightarrow{c}\). Then prove that \(\overrightarrow {a}\times \overrightarrow {b}=\overrightarrow {b}\times \overrightarrow {c}=\overrightarrow {c}\times \overrightarrow {a}.\)