document.write( "Question 309331: fi8nd the sum of the arithmetic series.-4,+1+6+11+........+91 \n" ); document.write( "

Algebra.Com's Answer #221244 by Fombitz(32388) $\"\"$ $\"About$

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$\"a1=-4\"$
\n" ); document.write( " $\"d=5\"$
\n" ); document.write( "The sum of an arithmetic series is,
\n" ); document.write( " $\"Sn=n%28%28a1%2Ban%29%2F2%29\"$
\n" ); document.write( "Solve for n using the final term.
\n" ); document.write( " $\"an=a1%2B%28n-1%29%2Ad=91\"$
\n" ); document.write( " $\"-4%2B%28n-1%29%285%29=91\"$
\n" ); document.write( " $\"5%28n-1%29=95\"$
\n" ); document.write( " $\"n-1=19\"$
\n" ); document.write( " $\"n=20\"$
\n" ); document.write( "Then
\n" ); document.write( " $\"S20=20%28%28-4%2B91%29%2F2%29\"$
\n" ); document.write( " $\"S20=870\"$ \n" ); document.write( "

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