document.write( "Question 1040203: 15+12+9+6...=-90 find number of terms
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Algebra.Com's Answer #655007 by Fombitz(32388) $\"\"$ $\"About$

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$\"a%5B1%5D=15\"$
\n" ); document.write( " $\"d=-3\"$
\n" ); document.write( " $\"S%5BN%5D=-90\"$
\n" ); document.write( "So in terms of $\"a%5B1%5D\"$ and $\"d\"$ , the sum is,
\n" ); document.write( " $\"S%5BN%5D=N%2Aa%5B1%5D%2B%28%28N%28N%2B1%29%29%2F2%29%2Ad\"$
\n" ); document.write( "Substituting,
\n" ); document.write( " $\"-90=N%2A15%2B%28%28N%28N%2B1%29%29%2F2%29%2A%28-3%29\"$
\n" ); document.write( " $\"-90=15N-%283%2F2%29%28N%28N%2B1%29%29\"$
\n" ); document.write( " $\"-180=15N-3%28N%5E2%2BN%29\"$
\n" ); document.write( " $\"-60=5N-%28N%5E2%2BN%29\"$
\n" ); document.write( " $\"N%5E2-4N-60=0\"$
\n" ); document.write( " $\"%28N-10%29%28N%2B6%29=0\"$
\n" ); document.write( "Only the positive value makes sense in this problem.
\n" ); document.write( " $\"N-10=0\"$
\n" ); document.write( " $\"N=10\"$
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