simple: $\frac{1}{2}$

more complex: $\frac{x+bad1}{x+2}$

text: $\frac{\text{number of bugs}}{\text{number of changes}}$

line thickness by keyword: $\genfrac{}{}{.05ex}{}{1}{2}$$\genfrac{}{}{}{}{1}{2}$$\genfrac{}{}{.2ex}{}{1}{2}$

numerator alignment: $\frac{x\hfill }{1+2+3+4+5}$ $\frac{x}{1+2+3+4+5}$ $\frac{\hfill x}{1+2+3+4+5}$

denominator alignment: $\frac{1+2+3+4+5}{x\hfill }$ $\frac{1+2+3+4+5}{x}$ $\frac{1+2+3+4+5}{\hfill x}$