MathRevolution
[GMAT math practice question]
If the terms of a sequence {A
n} are defined by A
n=\(\frac{n}{100}\), where \(n\) ranges over all the integers between \(101\) and \(200\), inclusive, what is the sum of all the A
n?
A. 150
B. 150.5
C. 151
D. 200
E. 201
The sum of an arithmetic series = (average)*(# of terms)
Average: (First Term + Last Term) ÷ 2
Number of terms: (Last Term - First Term) + 1
Methods (both are quick):
1) Use decimals Use this sequence's numbers in decimal form. Find the sum.
\(\frac{101}{100}=1.01\) Find the sum of 1.01, 1.02 . . 1.99, 2.00
Average:
\(\frac{(1.01+2.00)}{2} = \frac{3.01}{2}\) = 1.505
# of terms: (200 - 101) = 99, and (99 + 1) = 100 terms
Sum = (1.505)*(100) =
150.5Answer B
2)
forget about the fractions. Use only the integersFind the average of the integers. Divide by 100.
Multiply that value by # of terms in the fractional sequence. Why?
*The average of the just the integers, divided by 100,
equals the average of all \(\frac{A_{n}}{100}\)First, integers only.
Find the average of integers from 101 to 200, inclusive:
\(101 + 200 = \frac{301}{2} = 150.5\)Second, divide that average by 100
(the integers we just averaged are all divided by 100):
\(\frac{150.5}{100} = 1.505\) That's the average of this sequence with fractions
Third, number of terms in this sequence:
(# of terms) = (200-101= 99), 99 + 1 = 100 terms
Sum:
\((1.505*100)\) \(=150.5\)Answer B
*Try a small sample to prove the point. Say, e.g., we have this prompt's facts,
but the range is 1 to 3 inclusive. \(\frac{1}{100} = .01\)
Average:\((\frac{.01+.03}{2})=.02\)
Now try integers: average of 1 through 3 = \(\frac{1 + 3}{2} = 2\)
Divide that average by 100. \(\frac{2}{100} = .02\). That works.