Advertisements
Advertisements
प्रश्न
Find the sum of first 15 multiples of 8.
Advertisements
उत्तर १
The multiples of 8 are
8, 16, 24, 32…
These are in an A.P., having the first term as 8 and common difference as 8.
Therefore, a = 8
d = 8
S15 =?
`S_n = n/2[2a+(n-1)d]`
= `15/2[2(8)+(15-1)8]`
= `15/2[16+14(8)]`
= `15/2(16+112)`
= `(15(128))/2`
= 15 × 64
= 960
Therefore, the sum of the first 15 multiples of 8 is 960.
उत्तर २
Multiples of 8 are: 8, 16, 24, 32, ........... , Which form an A.P. with first term, a = 8 and common difference, d = 8
∵ Sum of nth term of A.P.
Sn = `n/2[2a + (n - 1)d]`
∴ S15 = `15/2 [2 xx 8 + (15 - 1) xx 8]`
= `15/2 [16 + 112]`
= `15/2 xx 128`
= 15 × 64
= 960
APPEARS IN
संबंधित प्रश्न
The houses in a row numbered consecutively from 1 to 49. Show that there exists a value of x such that sum of numbers of houses preceding the house numbered x is equal to sum of the numbers of houses following x.
Find the sum of first 20 terms of the following A.P. : 1, 4, 7, 10, ........
If the 3rd and the 9th terms of an AP are 4 and –8 respectively, which term of this AP is zero?
Ramkali saved Rs 5 in the first week of a year and then increased her weekly saving by Rs 1.75. If in the nth week, her week, her weekly savings become Rs 20.75, find n.
In an AP, given a = 2, d = 8, and Sn = 90, find n and an.
Show that a1, a2,..., an... form an AP where an is defined as below:
an = 9 − 5n
Also, find the sum of the first 15 terms.
Find the sum of n terms of an A.P. whose nth terms is given by an = 5 − 6n.
Which term of AP 72,68,64,60,… is 0?
If (2p – 1), 7, 3p are in AP, find the value of p.
Find the first term and common difference for the A.P.
0.6, 0.9, 1.2,1.5,...
The first term of an A. P. is 5 and the common difference is 4. Complete the following activity and find the sum of the first 12 terms of the A. P.
a = 5, d = 4, s12 = ?
`s_n = n/2 [ square ]`
`s_12 = 12/2 [10 +square]`
`= 6 × square `
` =square`
The sum of first 9 terms of an A.P. is 162. The ratio of its 6th term to its 13th term is 1 : 2. Find the first and 15th term of the A.P.
The first term of an A.P. is p and its common difference is q. Find its 10th term.
If `4/5` , a, 2 are three consecutive terms of an A.P., then find the value of a.
Q.6
The sum of the first three numbers in an Arithmetic Progression is 18. If the product of the first and the third term is 5 times the common difference, find the three numbers.
Find the common difference of an A.P. whose first term is 5 and the sum of first four terms is half the sum of next four terms.
If ₹ 3900 will have to be repaid in 12 monthly instalments such that each instalment being more than the preceding one by ₹ 10, then find the amount of the first and last instalment
Find the sum of first seven numbers which are multiples of 2 as well as of 9.
Show that the sum of an AP whose first term is a, the second term b and the last term c, is equal to `((a + c)(b + c - 2a))/(2(b - a))`
