If the sequence X(1), X(2), X(3),...,X(n),..is such that X(1) = 3 and X(n+1) = 2X(n) - 1 for n = 1, then X(20)-X(19) =
2^19
2^20
2^21
2^20 - 1
2^21 - 1
The ANS is A ,but how to solve it?
5-Day Free Trial
5-day free, full-access trial TTP
Available with Beat the GMAT members only code
MORE DETAILS
PS
This topic has expert replies
x(2) = 2(x(1)) -1 = 2(3) -1 = 5
As per the series...
x(1) = 2^1 + 1
x(2) = 2^2 + 1 and so on
so,
x(19) = 2^19 + 1 and
x(20) = 2^20 + 1
x(20) - x(19) = (2^20 + 1) - (2^19 + 1)
= 2^20 + 1 - 2^19 - 1
= 2^20 - 2^19
= 2^19
Ans A
As per the series...
x(1) = 2^1 + 1
x(2) = 2^2 + 1 and so on
so,
x(19) = 2^19 + 1 and
x(20) = 2^20 + 1
x(20) - x(19) = (2^20 + 1) - (2^19 + 1)
= 2^20 + 1 - 2^19 - 1
= 2^20 - 2^19
= 2^19
Ans A
-
samirpandeyit62
- Master | Next Rank: 500 Posts
- Posts: 460
- Joined: Sun Mar 25, 2007 7:42 am
- Thanked: 27 times
As per the series...
x(1) = 2^1 + 1
x(2) = 2^2 + 1 and so on
&
x(20) - x(19)
= 2x(19 ) - 1 - x(19) = x(19) -1
so reqd val is 2^19 -1 + 1 = 2^19
x(1) = 2^1 + 1
x(2) = 2^2 + 1 and so on
&
x(20) - x(19)
= 2x(19 ) - 1 - x(19) = x(19) -1
so reqd val is 2^19 -1 + 1 = 2^19
Regards
Samir
Samir
