Prove that the syntax is equivalent
An intuitive approach towards the difference is understanding that the productions of syntax #1 add new material to the left of existing symbols whereas those of syntax #2 do so on the right-hand side.


syntax #1:


derived string of terminals: `  T    + T   + T  ...  + T   + T `
derivation step:             `[ n ] [n-1] [n-2]     [ 2 ] [ 1 ]`

syntax #2 (step #n is the application of rule 3 and produces no terminal) :


derived string of terminals: `  T    + T   + T  ...  + T   + T `
derivation step:             `[ 1 ] [ 2 ] [ 3 ]     [n-2] [n-1]`



for a rigorous proof note that under both syntaxes, each derivation leads to an initial T followed by sequence of  + T. for the grammars at hand this can be proven by induction over the number of rule applications.