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 Expert Answer
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 ANSWERED BY EXPERT
CBSE 10 - Maths
Asked by bkhaiwangkonyak | 07 Mar, 2023, 04:35: PM ANSWERED BY EXPERT
CBSE 10 - Maths
the sum of fourth and eight terms of an A.P is 54 and their product is 665. find the sum of the first thirty terms of an A.P​ Asked by sakshinde95 | 23 Jan, 2023, 05:50: PM ANSWERED BY EXPERT
CBSE 10 - Maths
Asked by d7016880266 | 20 Jan, 2023, 08:14: PM ANSWERED BY EXPERT
CBSE 10 - Maths
maths doubt Asked by dixitayush414 | 04 Jan, 2023, 09:03: PM ANSWERED BY EXPERT
CBSE 10 - Maths ANSWERED BY EXPERT ANSWERED BY EXPERT ANSWERED BY EXPERT ANSWERED BY EXPERT ANSWERED BY EXPERT