Arithmetic progression

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T_A_M: Senior | Next Rank: 100 Posts; Posts: 43; Joined: Sun Apr 19, 2009 2:02 am; Thanked: 2 times

by T_A_M » Thu May 07, 2009 12:41 pm

Hi
Please can someone help me with this problem.
I know the formula.

a + (n-1) d

In this case a is 23 and d is -3

But what is n?

How do you approach this?

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Source: Beat The GMAT — Problem Solving | Thu May 07, 2009 12:41 pm

vitaly: Junior | Next Rank: 30 Posts; Posts: 18; Joined: Tue May 05, 2009 11:03 am

by vitaly » Fri May 08, 2009 7:56 pm

I know the formula, but I don't know what this formula is for

Is it to find n-th member of arithmetic progression?

a is the first member
d is the difference between a_i, a_(i-1)

for example
0, 2, 4, 8, 10, ...

a = 0, d = 2
So 5-th member a_5 = a+5d = 10

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