Math notation for conditions, branches, ranges, switches

Question

How to write such logic in a short math notation?

Tax for the income is 9% for the part that's below €750/month and 11% for part that's above.

Answer 1

Piecewise: $$ t(x)= \begin{cases} 0.09x &\text{if $x \le 750$}\\ 0.09(750)+0.11(x-750) &\text{otherwise} \end{cases} $$ Shorter: $$t(x) = 0.09x+0.02\max(x-750,0)$$