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.
The sum of three numbers in A.P. is –3, and their product is 8. Find the numbers
How many terms of the series 54, 51, 48, …. be taken so that their sum is 513 ? Explain the double answer
Which term of the progression 20, 19`1/4`,18`1/2`,17`3/4`, ... is the first negative term?
Find the sum of all integers between 50 and 500, which are divisible by 7.
If the 8th term of an A.P. is 37 and the 15th term is 15 more than the 12th term, find the A.P. Also, find the sum of first 20 terms of A.P.
The 24th term of an AP is twice its 10th term. Show that its 72nd term is 4 times its 15th term.
Find the sum of first n even natural numbers.
If (2p +1), 13, (5p -3) are in AP, find the value of p.
Write an A.P. whose first term is a and the common difference is d in the following.
a = 10, d = 5
Choose the correct alternative answer for the following question .
In an A.P. first two terms are –3, 4 then 21st term is ...
If the sum of a certain number of terms starting from first term of an A.P. is 25, 22, 19, ..., is 116. Find the last term.
If the sum of n terms of an A.P. be 3n2 + n and its common difference is 6, then its first term is ______.
Q.12
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.
In an A.P., the sum of its first n terms is 6n – n². Find is 25th term.
In an A.P. a = 2 and d = 3, then find S12
Find the sum of first seven numbers which are multiples of 2 as well as of 9.
Kanika was given her pocket money on Jan 1st, 2008. She puts Rs 1 on Day 1, Rs 2 on Day 2, Rs 3 on Day 3, and continued doing so till the end of the month, from this money into her piggy bank. She also spent Rs 204 of her pocket money, and found that at the end of the month she still had Rs 100 with her. How much was her pocket money for the month?
