Question 663230
Proving Theorem 2-1

 Given: <{{{1}}} and <{{{2}}} are supplementary
<{{{3}}} and <{{{4}}} are supplementary
<{{{2}}} is congruent to <{{{4}}}

Prove: <{{{1}}} is congruent to <{{{3}}} 


Paragraph Proof: 


By the Angle Addition Postulate, <{{{1}}} +<{{{2}}} = {{{180}}} and <{{{3}}} + <{{{4}}}  = {{{180}}}. 

By substitution, <{{{1}}} +<{{{2}}} = <{{{3}}} + <{{{4}}}. 

It is given that <{{{2}}} is congruent to <{{{4}}}.

By substitution, <{{{1}}} +<{{{2}}} = <{{{3}}} + <{{{2}}}. 

Subtract <{{{2}}} from each side. 

You get <{{{1}}} =  <{{{3}}}, or <{{{1}}} is congruent to <{{{3}}}.