## Important Concepts and Formulas - Sequence and Series

Comments(31)

reshma
27 Oct 2014 9:21 AM

what is the formula to find the product of gp

PARVEEN
18 Oct 2014 4:09 PM

sum of three numbers in ap is 21 and if one addes to third no and one deducted from 2nd no the series become gp what are the numbers

Raj
23 Oct 2014 10:31 PM

Let numbers be (a-d), a and (a+d)

sum = 3a = 21

a = 7

So the numbers are (7-d), 7 , (7+d)

Given that (7-d), 6, (8+d) are in GP

6

36 = 56 + 7d - 8d -d

d2 + d - 20 = 0

(d+5)(d-4)=0

d = -5 or 4

so numbers are 12, 7, 2

OR

numbers are 3, 7, 11

sum = 3a = 21

a = 7

So the numbers are (7-d), 7 , (7+d)

Given that (7-d), 6, (8+d) are in GP

6

^{2}= (7-d)(8+d)36 = 56 + 7d - 8d -d

^{2}d2 + d - 20 = 0

(d+5)(d-4)=0

d = -5 or 4

so numbers are 12, 7, 2

OR

numbers are 3, 7, 11

vivek
11 Oct 2014 11:42 AM

what is answer for this question

if the 9th term ap is zero then t29/t19=k. where k=?

Raj
16 Oct 2014 10:23 PM

9th term = 0

a + (9-1)d = 0

a + 8d = 0

a = -8d

Anirudh Mishra
16 Oct 2014 10:16 PM

VIVEK, answer for your question is 2

Harshit
04 Sep 2014 1:01 PM

How to you find the sum of a series if the difference of terms of the series are in ap.

anirudh
30 Sep 2014 11:19 PM

if common difference is in A.P. eg:1 2 4 7 11 16 ..... nterms.

S= 1 + 2 + 4 + 7 + 11 + .......... +Tn

S = 1 + 2 + 4 + 7 + ...............+Tn-1 + T

_{N}SUBTRACT :

0= 1- ( 1 + 2+ 3+ 4+ ........ N-1 TERM) - Tn term

Tn =1+ n(n-1)/2

NOW FIND Sn .

somesh chandra
07 Jul 2014 5:28 PM

Q. find the ans srise

0,8,54,192,500,?

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