document.write( "<A HREF=/cgi-bin/jump-to-question.mpl?question=430148>Question 430148</A>: <I><SPAN STYLE=\"word-break: break-all;\"> Can you please write me a formal geometry of 'the consecutive angles of a parallelogram are supplementary'?  I know that they are, but I just started writing proofs and I can't think of how to explain something so basic... Thank you so much! </SPAN>\n" );
document.write( "</I><HR><TABLE width=100%><TR><TD><B><A HREF=http://www.algebra.com/>Algebra.Com's</A> Answer #298747 by </B><A HREF=/tutors/aboutme.mpl?userid=richard1234><B>richard1234(7193)</B></A><img src=\"/images/stars/stars-13.gif\" alt=\"\" >&nbsp;<A HREF=/tutors/aboutme.mpl?userid=richard1234><IMG SRC=http://www.algebra.com/images/aboutme-small.gif BORDER=0 alt=\"About Me\" VALIGN=MIDDLE></A>&nbsp;<IMG SRC=http://www.algebra.com/cgi-bin/show-face.mpl?user=richard1234><BR>You can <A HREF=\"/cgi-bin/embed-solution.mpl?show=1&solution=298747\">put this solution on YOUR website!</A><BR><SPAN STYLE=\"word-break: break-all;\"> Use the fact that the sides of the parallelogram are parallel. One of Euclid's postulates says that if two lines are parallel, then its corresponding angles add up to 180 degrees. It is a little difficult to formalize it because such a proof relies on a postulate, something that cannot rigorously be proven. </SPAN>\n" );
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