Cho tam giác ABC có AB = c, CA = b, BC = a . Tính \(\overrightarrow {AB} .\overrightarrow {BC} \) theo abc

A. \(\frac{1}{2}\left( {{b^2} + {c^2} - {a^2}} \right)\)

B. \(\frac{1}{2}\left( {{a^2} - {b^2} - {c^2}} \right)\)

C. \(\frac{1}{2}\left( {{a^2}+ {b^2} - {c^2}} \right)\)

D. \(\frac{1}{2}\left( {{b^2} - {c^2} - {a^2}} \right)\)