Complex number operations: telling Mathematica variables are real

Question

I want to do Conjugate[a + b*I], but when I do that, the solution is Conjugate[a] - I*Conjugate[b]; when for me, a and b are reals.

I want to obtain the following expresion : a-b*I

The same problem exists with the function Abs.

Answer 1

Conjugate[a + b*I]//ComplexExpand

or

Refine[Conjugate[a + b*I], {a, b} \[Element] Reals]