a)

$\frac{1}{2}+\frac{1}{4}=\frac{2}{4}+\frac{1}{4}=\frac{3}{4}$

b)

$\frac{3}{4}+\frac{1}{2}=\frac{3}{4}+\frac{2}{4}=1\frac{1}{4}$