Advertisements
Advertisements
प्रश्न
Sum of the first 20 terms of an AP is −240, and its first term is 7. Find its 24th term ?
उत्तर
First term of the A.P. (a) = 7; sum of first 20 terms = −240.
The sum of first n terms of an AP,`S_n= n/2[2a+(n-1)d]`, where a is the first term and d is the common difference.
`therefore S_n=20/2[2xx7+(20-1)d]=240`
`rArr10[14+19d]=-240`
`rArr14+19d=-24`
`rArr19d=-38`
`rArrd=-2`
Now, 24th term of the AP, `a_24=a+(24-1)d`
On putting respective values of a and d, we get
a24 + 7 + 23 × (− 2) = 7 − 46 = − 39
Hence, 24th term of the given AP is −39.
APPEARS IN
संबंधित प्रश्न
Write the first five terms of the following sequences whose nth terms are:
an = (−1)n, 2n
How many numbers of two digit are divisible by 3?
Show that each of the progressions given below is an AP. Find the first term, common difference and next term of each.
(iv) 2, 8, 18, 32,..........
20th term of the AP -5, -3, -1, 1, is ______.
The 6th term from the end of the AP: 5, 2, -1, -4, …., -31, is ______.
The 10th term from the end of the A.P. -5, -10, -15,…, -1000 is ______.
Justify whether it is true to say that the following are the nth terms of an AP.
2n – 3
If 7 times the 7th term of an AP is equal to 11 times its 11th term, then its 18th term will be ______.
Justify whether it is true to say that the following are the nth terms of an AP.
3n2 + 5
Fill in the blank in the following table, given that a is the first term, d the common difference, and an nth term of the AP:
| a | d | n | an |
| -18 | ______ | 10 | 0 |
