document.write( "<A HREF=/cgi-bin/jump-to-question.mpl?question=626854>Question 626854</A>: <I><SPAN STYLE=\"word-break: break-all;\"> A friend claims that by writing the product 19x21 as<BR>\n" );
document.write( "(20-1)(20 + 1), the product can be computed mentally. How is this possible? </SPAN>\n" );
document.write( "</I><HR><TABLE width=100%><TR><TD><B><A HREF=http://www.algebra.com/>Algebra.Com's</A> Answer #394463 by </B><A HREF=/tutors/aboutme.mpl?userid=reviewermath><B>reviewermath(1029)</B></A><img src=\"/images/stars/stars-12.gif\" alt=\"\" >&nbsp;<A HREF=/tutors/aboutme.mpl?userid=reviewermath><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=reviewermath><BR>You can <A HREF=\"/cgi-bin/embed-solution.mpl?show=1&solution=394463\">put this solution on YOUR website!</A><BR><SPAN STYLE=\"word-break: break-all;\"> The product of the sum and difference is equal to the difference between the squares. <BR>\n" );
document.write( "In symbol: <IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=%28a%2Bb%29%28a-b%29+=+a%5E2+-+b%5E2+ ALIGN=MIDDLE  ALT=\"%28a%2Bb%29%28a-b%29+=+a%5E2+-+b%5E2+\"   DISABLED_style= \"max-width:90%; height:auto;\" >. <BR>\n" );
document.write( "Let a = 20 and b = 1 so <IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=%2820%2B1%29%2820-1%29+=+20%5E2+-+1%5E2+=+400+-+1+=+highlight%28399%29+ ALIGN=MIDDLE  ALT=\"%2820%2B1%29%2820-1%29+=+20%5E2+-+1%5E2+=+400+-+1+=+highlight%28399%29+\"   DISABLED_style= \"max-width:90%; height:auto;\" > </SPAN>\n" );
document.write( "</TD></TR></TABLE>\n" );