Difference between revisions of "2007 iTest Problems/Problem 5"

(See Also)
(category)
 
(One intermediate revision by one other user not shown)
Line 17: Line 17:
  
 
==See Also==
 
==See Also==
{{iTest box|year=2007|num-b=4|num-a=6|}}
+
{{iTest box|year=2007|num-b=4|num-a=6}}
 +
 
 +
[[Category:Introductory Algebra Problems]]

Latest revision as of 19:46, 14 January 2008

Problem

Compute the sum of all twenty-one terms of the geometric series \[1 + 2 + 4 + 8 + \ldots + 1048576\].

$\mathrm{(A)}\,2097149\quad\mathrm{(B)}\,2097151\quad\mathrm{(C)}\,2097153\quad\mathrm{(D)}\,2097157\quad\mathrm{(E)}\,2097161$

Solution

Those are powers of 2. So it would make sense to multiply the series by 2:

$x=1 + 2 + 4 + 8 + \ldots + 1048576$

$2x=2 + 4 + 8 + \ldots + 1048576 + 2097152$

Subtracting,

$x=2097152-1=2097151 \Rightarrow \mathrm {(B)}$

See Also

2007 iTest (Problems, Answer Key)
Preceded by:
Problem 4
Followed by:
Problem 6
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 TB1 TB2 TB3 TB4