Saved Bookmarks
| 1. |
The sum of first m terms of an AP is `(4m^2-m)` If its nth term is 107, find the value of n. Also find the 21st term of this AP |
|
Answer» Correct Answer - n = 14, `T_(21) = 163` `S_(m) = (4m^(2) - m) rArr S_(m-1) = 4(m-1)^(2) - (m-1) = (4m^(2) -9m + 5)` `therefore T_(m) = (S_(m) - S_(m-1)) = (4m^(2)-m) - (4m^(2) -9m + 5) = (8m - 5)` `rArr T_(n) = (8n-5) rArr 8n - 5 = 107 rArr n = 14` `therefore T_(21) = (8 xx 21 - 5) = 163` |
|