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| 1. |
In an Ap , the sum of frist n term is 3n^2/2+13n/2 . Find the 25th term |
| Answer» {tex}S_n = \\frac { 3 n ^ { 2 } } { 2 } + \\frac { 13 n } { 2 }{/tex}{tex}{ \\mathrm { S } _ { n } = \\frac { 3 n ^ { 2 } + 13 n } { 2 } }{/tex}an\xa0= Sn\xa0- Sn-1or, a25\xa0= S25\xa0- S24{tex}= \\frac { 3 ( 25 ) ^ { 2 } + 13 ( 25 ) } { 2 } - \\frac { 3 ( 24 ) ^ { 2 } + 13 ( 24 ) } { 2 }{/tex}=\xa0{tex}\\frac 12{/tex}[3(252\xa0- 242)\xa0+13(25 - 24)]= {tex}\\frac { 1 } { 2 } {/tex}[3 (625 - 576) + 13 (1)]=\xa0{tex}\\frac { 1 } { 2 } ( 3 \\times 49 + 13 ){/tex}= {tex}\\frac { 1 } { 2 } ( 147 + 13 ){/tex}= {tex}\\frac { 1 } { 2 } ( 160 ){/tex}= 80 | |