Answer
$c=\frac{1}{2}a+\frac{1}{2}b$,
$d=\frac{1}{2}b-\frac{1}{2}a$
Work Step by Step
As per parallelogram law: $a+b=2c$
Divide both sides by scalar $2$, we get
$c=\frac{a+b}{2}$
$c=\frac{1}{2}a+\frac{1}{2}b$
From the figure we can see that: $a+2d=b$
$2d=b-a$
Divide both sides by scalar $2$, we get
$d=\frac{b-a}{2}$
$d=\frac{1}{2}b-\frac{1}{2}a$
