BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem

Question about using Equations

Problem Solving — algebra and arithmetic (GMAT Focus Edition)
This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
Senior | Next Rank: 100 Posts
Posts: 38
Joined: Fri Aug 17, 2012 11:28 am
Thanked: 2 times

Question about using Equations

by avada » Mon Sep 03, 2012 3:53 pm
((n)(n+1))/2
It is my understanding that this equation is used to find the sum of n numbers. so if we are trying find 4! this equations gives use 10.

What is the equation if I wanted to find the summation of all the odd numbers or even numbers for a set?

I have seen (n/2)(2a+(n-1)d) but I am not sure what it is for.

Thanks

(Also any example problems where this could be used would be awesome)
Top
Source: — Quantitative Reasoning |
User avatar
MBA Admissions Consultant
Posts: 2279
Joined: Fri Nov 11, 2011 7:51 am
Location: New York
Thanked: 660 times
Followed by:266 members
GMAT Score:770

by Jim@StratusPrep » Mon Sep 03, 2012 5:00 pm
You can always take the average of the first and last term and multiply that by the number of terms:


1,3,5,7,9

(1+9)/2 x 5 = 5 x 5 = 25
GMAT Answers provides a world class adaptive learning platform.
-- Push button course navigation to simplify planning
-- Daily assignments to fit your exam timeline
-- Organized review that is tailored based on your abiility
-- 1,000s of unique GMAT questions
-- 100s of handwritten 'digital flip books' for OG questions
-- 100% Free Trial and less than $20 per month after.
-- Free GMAT Quantitative Review

Image
Top

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 16207
Joined: Mon Dec 08, 2008 6:26 pm
Location: Vancouver, BC
Thanked: 5254 times
Followed by:1268 members
GMAT Score:770

by Brent@GMATPrepNow » Tue Sep 04, 2012 4:31 am
The sum of odd numbers from 1 to some value is nice in that the sum is always the square of the number of odd values.
examples
1 = 1 (= 1^2)
1+3 = 4 (= 2^2)
1+3+5 = 9 (= 3^2)
1+3+5+7 = 16 (= 4^2)
etc.

For the sum of even numbers, we can use the n(n+1)/2 formula. All we need to do is factor out the 2 first.
For example:
2+4+6+8+10+12 = 2(1+2+3+4+5+6)
From here we can use the formula to find the sum of the integers from 1 to 6, which equal (6)(6+1)/2 = 21
So, 2+4+6+8+10+12 = 2(1+2+3+4+5+6) = 2(21) = 42

I hope that helps.

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
Image
Top
Post new topic Post Reply
• Page 1 of 1