document.write( "Question 1086929: 20.Find the sum of this series: 87 + 72 + 57 + 42 + …; 20th term. \n" ); document.write( "

Algebra.Com's Answer #701197 by Boreal(15235) $\"\"$ $\"About$

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a1=87 common difference is -15
\n" ); document.write( "a20=a1+(19)(-15)=87-285=-198
\n" ); document.write( "sum is n(a1+an)/2
\n" ); document.write( "=10(-111)
\n" ); document.write( "=-1110
\n" ); document.write( "87
\n" ); document.write( "72
\n" ); document.write( "57
\n" ); document.write( "42
\n" ); document.write( "27
\n" ); document.write( "12--positive add to 297
\n" ); document.write( "-3
\n" ); document.write( "-18
\n" ); document.write( "-33
\n" ); document.write( "-48
\n" ); document.write( "-63
\n" ); document.write( "-78
\n" ); document.write( "-93
\n" ); document.write( "-108
\n" ); document.write( "-123
\n" ); document.write( "-138
\n" ); document.write( "-153
\n" ); document.write( "-168
\n" ); document.write( "-183
\n" ); document.write( "-198--negative add to -1407
\n" ); document.write( "-1110 is sum. \n" ); document.write( "

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