You need a couple more statements.  Also, if you 
use < for an angle followed by a letter (not a number) you
must skip a space after < , otherwise the program will think 
it's an HTML tag and will delete what follows it. It's best 
to use the symbol ∠ for an angle which is gotten by 
typing this code:

.  
 

Given : m∠1 = m∠3 and m∠2 = m∠4
Prove : m∠ABC = m∠DEF




Statements                Reasons 

1 m∠1 = m∠3               Given

2 m∠2 = m∠4               Given

3 m∠1 + m∠2 = m∠3 + m∠2  Equals (in step 1) added to equals 
                          (in step 2) gives equals.

4 m∠1 + m∠2 = m∠3 + m∠4  equals (in step 2) may be substituted
                          for each other (in step 3)

5 m∠1 + m∠2 = m∠ABC      a whole is equal to the sum of its parts 

6 m∠3 + m∠4 = m∠DEF      a whole is equal to the sum of its parts

7 m∠ABC = m∠DEF          equals (in steps 5 and 6) may be substituted 
                          for each other (in step 4).  

Edwin