document.write( "Question 626854: A friend claims that by writing the product 19x21 as
\n" ); document.write( "(20-1)(20 + 1), the product can be computed mentally. How is this possible? \n" ); document.write( "
Algebra.Com's Answer #394462 by jim_thompson5910(35256)\"\" \"About 
You can put this solution on YOUR website!
(20-1)(20 + 1) = 20^2 - 1^2 = 400 - 1 = 399\r
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\n" ); document.write( "\n" ); document.write( "So 19 x 21 = 399\r
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