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### assign variable value

Hi!

I'm just getting started with Sage and therefore my question may sound really stupid (and nevertheless I couldn't find an answer here). I have a matrix with both symbolic entries and numerical values, let's say:

a,b=var('a,b')
M=matrix([[1,a], [0,b]])


Now I want do assign a value to a:

a=0


However if I print M to the screen it still says a in the matrix and not 0.

You can clearly see that I'm really not used to Sage, so what's the problem here? What should I do if I want to assign a fixed value to a variable which is also used in the calculations where this variable appears?

Thank's for the answers, I know this will sound highly trivial for you!

Philipp