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Important Concepts and Formulas - Sequence and Series

 



Comments(33)


priya 16 Nov 2014 7:13 PM
In how many ways can letters of English alphabet be arranged so that there are 7 letters between the letters a and b and no letter is repeated??
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Raj 21 Nov 2014 12:37 AM
Assuming that all arrangements are done by taking all 26 alphabets at a time

Removing A and B, there are 24 letters. From these letters, 7 alphabets can be arranged between A and B in 24P7 ways

A and B can be arranged in 2! ways

Remaining alphabets = 17
Consider (A)(7 letters)(B) as a single alphabet now
Lets consider this as a total of 18  alphabets . These alphabets can be arranged among themselves in 18! ways

Required number of ways = 24P7* 2! * 18!
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reshma 27 Oct 2014 9:21 AM
what is the formula to find the product of gp
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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
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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
62 = (7-d)(8+d)
36 = 56 + 7d - 8d -d2
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

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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=?
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Raj 16 Oct 2014 10:23 PM
9th term = 0
a + (9-1)d = 0
a + 8d = 0
a = -8d

k = [a + (29-1)d] / [a + (19-1)d] = (-8d+ 28d) / (-8d + 18d) = 20d/10d = 2
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Anirudh Mishra 16 Oct 2014 10:16 PM
VIVEK, answer for your question is 2
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seenu 06 Sep 2014 7:40 PM
i like maths and i love maths
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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.
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