(a)  Formula: 1 Mark
      Steps: 1 Mark
      Answer: 1 Mark
(b) Answer: 1 Mark
(a) To find the length of the diagonal, d, of each rectangle, use Pythagoras Theorem.
For A, d $=\sqrt{(}{1}^2+2^2)=\sqrt{(}1+4)=\sqrt{(}5)$
For B, d $=\sqrt{(}{2}^2+3^2)=\sqrt{(}4+9)=\sqrt{(}13)$
For C, d $=\sqrt{(}{3}^2+4^2)=\sqrt{(}9+16)=\sqrt{(}25)$
For D, d $=\sqrt{(}{4}^2+5^2)=\sqrt{(}16+25)=\sqrt{(}41)$
Of these, only $\sqrt{(}25)=5$ is rational.
The length of the diagonal of rectangle C is rational.
The length of the diagonal of rectangle A, B and D are irrational
(b)  $\frac{5}{13}-\frac{-8}{26}$
$\Rightarrow \frac{5}{13}+\frac{8}{26}$
$\Rightarrow \frac{10+8}{26}$
$\Rightarrow \frac{18}{26}=\frac{9}{13}$