ABCD is a parallelogram with diagonals AC and BD. $\vec{AD}=$

• a$\frac{1}{2}(\vec{AC}-\vec{BD})$
• b$\frac{1}{2}(\vec{AC}+\vec{BD})$
• c$\vec{AC}+\vec{BD}$
• d$\vec{AC}-\vec{BD}$