Advertisements
Advertisements
प्रश्न
In an AP if a = 1, an = 20 and Sn = 399, then n is ______.
पर्याय
19
21
38
42
उत्तर
In an AP if a = 1, an = 20 and Sn = 399, then n is 38.
Explanation:
∵ Sn = `n/2[2a + (n - 1)d]`
339 = `n/2[2 xx 1 + (n - 1)d]`
798 = 2n + n(n – 1)d ...(i)
And an = 20
⇒ a + (n – 1)d = 20 ...[∵ an = a + (n – 1)d]
⇒ 1 + (n – 1)d = 20
⇒ (n – 1)d = 19 ...(ii)
Using equation (ii) in equation (i), we get
798 = 2n + 19n
⇒ 798 = 21n
∴ n = `798/21` = 38
APPEARS IN
संबंधित प्रश्न
Find the sum of the first 25 terms of an A.P. whose nth term is given by an = 7 − 3n
How many three-digit numbers are divisible by 9?
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`
If the seventh term of an A.P. is \[\frac{1}{9}\] and its ninth term is \[\frac{1}{7}\] , find its (63)rd term.
Find the sum (−5) + (−8)+ (−11) + ... + (−230) .
Find the sum of n terms of the series \[\left( 4 - \frac{1}{n} \right) + \left( 4 - \frac{2}{n} \right) + \left( 4 - \frac{3}{n} \right) + . . . . . . . . . .\]
Mrs. Gupta repays her total loan of Rs. 1,18,000 by paying installments every month. If the installments for the first month is Rs. 1,000 and it increases by Rs. 100 every month, What amount will she pays as the 30th installments of loan? What amount of loan she still has to pay after the 30th installment?
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?
Find the sum of those integers between 1 and 500 which are multiples of 2 as well as of 5.
If 7 times the seventh term of the AP is equal to 5 times the fifth term, then find the value of its 12th term.
