Math Is Fun Forum

Ani

Guest

arithmetic series

I've been sitting for a few hours thinking how to figure out the question below, could you help me solve this in a step-by-step format?

The Question;
The sum of the first five terms of an arithmetic series is 85. The sum of the first six terms is 123. What are the first four terms of the series?

Thank you for any help

Fruityloop

Member

Registered: 2009-05-18

Posts: 143

Re: arithmetic series

Use the difference between the sums to head towards an answer.

Ani

Guest

Re: arithmetic series

Thank you

Bob

Administrator

Registered: 2010-06-20

Posts: 10,643

Re: arithmetic series

hi Ani

Welcome to the forum.

From that information and with subtraction, you can say what the sixth term is.

There are two formulas that can be used:

and

So you can make two simultaneous equations in a and d.

Bob

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Children are not defined by school ...........The Fonz
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Sometimes I deliberately make mistakes, just to test you!  …………….Bob