document.write( "Question 998451: the fifth abd fifteenth term of an AP is 13 and 17 .find the sum of first 21 term \n" ); document.write( "

Algebra.Com's Answer #616216 by mananth(16946) $\"\"$ $\"About$

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1 a + 4 d = 13 .............1
\n" ); document.write( "Total value
\n" ); document.write( "1 a + 14 d = 17 .............2
\n" ); document.write( "Eliminate d
\n" ); document.write( "multiply (1)by -7
\n" ); document.write( "Multiply (2) by 2
\n" ); document.write( "-7 a -28 d = -91
\n" ); document.write( "2 a + 28 d = 34
\n" ); document.write( "Add the two equations
\n" ); document.write( "-5 a = -57
\n" ); document.write( "/ -5
\n" ); document.write( "a = 11
\n" ); document.write( "plug value of a in (1)
\n" ); document.write( "1 a + 4 d = 13
\n" ); document.write( "11 + 4 d = 13
\n" ); document.write( " 4 d = 13 -11
\n" ); document.write( " 4 d = 2
\n" ); document.write( " d = 1/2
\n" ); document.write( " \r
\n" ); document.write( "\n" ); document.write( "a= 11, d= 1/2 , n=15\r
\n" ); document.write( "\n" ); document.write( "Find $\"S%5Bn%5D+\"$ \r
\n" ); document.write( "\n" ); document.write( " $\"S%5Bn%5D+=+%28n%2F2%29%282a%2B%28n-1%29d%29%29\"$ \r
\n" ); document.write( "
\n" ); document.write( "\n" ); document.write( "Plug in the values to get $\"S%5B15%5D\"$ = 217.5
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