document.write( "Question 1086929: 20.Find the sum of this series: 87 + 72 + 57 + 42 + …; 20th term. \n" ); document.write( "
Algebra.Com's Answer #701206 by MathTherapy(10552)\"\" \"About 
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\n" ); document.write( "20.Find the sum of this series: 87 + 72 + 57 + 42 + …; 20th term.
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Use the sum-of-an-AP formula: \"matrix%281%2C3%2C+S%5Bn%5D%2C+%22=%22%2C+%28n%2F2%29%282a%5B1%5D+%2B+%28n+-+1%29d%29%29\", with:
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document.write( "\"S%5Bn%5D\" = Sum of \"n\" terms (Unknown, in this case)
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document.write( "\"n\"  = Number of terms (20, in this case)
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document.write( "\"a%5B1%5D\" = 1st term (87, in this case)
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document.write( "\"d\"  = Common difference (- 15, in this case)\r
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document.write( "Substitute these values and you should get: \"highlight_green%28-+%221%2C110%22%29\" as the sum. \n" );
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