x के लिए हल कीजिए:
` 5+ 13+ 21+ ..........+x=2139`

Explanation

यहाँ ` a= 5,d =13 -5 =8`
माना ` " " Tn=x`
` therefore " " S_n =2139`
` therefore " " (n)/(2) [2xx 5+ (n-1) 8] =2139`
` rArr " " n(5+4n -4 )= 2139 " " rArr " " 4n^(2) +n -2139=0`
`rArr " " 4n^(2) -92n+ 93n -2139 =0" " rArr " " (n-23 )(4n +93 )=0`

`rArr " " 4n^(2) -92n+ 93n -2139 =0" " rArr " " (n-23 )(4n +93 )=0`
` therefore " " n=23` या `" " n=-(93)/(4)`
` because nlt 0," " therefore n=23 ` `" " n=-(93)/(4)`
` because nlt 0," " therefore n=23 `
` therefore " " x=T_n =a+(n-1 )d =5 +(23-1 ) (8)`
` rArr " " x=181 `