Request a call back

an AP has 21 terms. the sum of it's 10th, 11th,12th terms is 129 and the sum of the last three terms is 237. find the AP.
Asked by anukeshd | 09 Mar, 2013, 12:08: AM
Answer : Given : an AP has 21 terms. the sum of it's 10th, 11th,12th terms is 129 and the sum of the last three terms is 237
To find : AP

Let a and d be the first term and common difference of the given A.P.
10 + a 11 + a 12 = 129
=> (a + 9d) + (a + 10d) + (a + 11d) = 129 [an = a + (n – 1)d]
=> 3a + 30d = 129
=> a + 10d = 43 … (1)
19 + a 20 + a 21 = 237
=> (a + 18d) + (a + 19d) + (a + 20d) = 237
=> 3a + 57d = 237
=> a + 19d = 79 … (2)
Solving (1) and (2), we get
a + 19d – a – 10d = 79 – 43
=> 9d = 36
=> d = 4
When d = 4, we get
a + 10 * 4 = 43
=> a = 43 – 40 = 3
Thus, the given A.P. is 3, 7, 11, 15… Answer
Answered by | 09 Mar, 2013, 12:48: AM

## Concept Videos

CBSE 10 - Maths
Asked by paresh0311 | 19 Mar, 2023, 12:26: AM
CBSE 10 - Maths
Asked by bkhaiwangkonyak | 07 Mar, 2023, 04:35: PM
CBSE 10 - Maths
Asked by sakshinde95 | 23 Jan, 2023, 05:50: PM
CBSE 10 - Maths
Asked by d7016880266 | 20 Jan, 2023, 08:14: PM
CBSE 10 - Maths
Asked by dixitayush414 | 04 Jan, 2023, 09:03: PM