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**Ani****Guest**

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: 135

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

**The eclipses from Algol come further apart in time when the Earth is moving away from Algol and closer together in time when the Earth is moving towards Algol, thereby proving that the speed of light is variable and that Einstein's Special Theory of Relativity is wrong.**

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**Ani****Guest**

Thank you

**bob bundy****Moderator**- Registered: 2010-06-20
- Posts: 7,661

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

Children are not defined by school ...........The Fonz

You cannot teach a man anything; you can only help him find it within himself..........Galileo Galilei

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