Posted by **Crystal** on Saturday, January 12, 2013 at 4:51pm.

Determine the sum of the first 20 terms of the arithmetic sequence in which ...

f) the 7th term is 43 and the 13th term is 109

I know you can make two equations :

43 = a + 6d and 109 = a + 12 d

And subtract them to get d = 11

But I know there's another way to get d, that you can just quickly do on your calculator. But I can't remember how to do it! It had something to do with dividing the number of terms or something...? I could probably figure it out after some trial and error but I just want to make sure I have the proper method down, so if someone could explain a clear and concise method to me, I would really appreciate it! I know it's really simple... I'm just so burned out right now. Too much studying, too much stress!

- Math -
**Reiny**, Saturday, January 12, 2013 at 5:34pm
There are 6 terms from term7 to term13

and the difference in their values is 109-43 = 66

so the common difference is 66/6 = 11

but, ... , notice that you are actually doing the same calculations you would do if you solved the two equations, so nothing gained.

- Math -
**Crystal**, Saturday, January 12, 2013 at 5:45pm
I think this way is a lot faster and easier to do by just quickly typing it all into the calculator, so thank you!

## Answer this Question

## Related Questions

- Precalculus - The 8th term in an arithmetic sequence is 5, and the sum of the ...
- Math *URGENT - Please give the answers and solutions for each. 1.If the second ...
- Maths - 1..The first 2 terms of a geometric progression are the same as the ...
- Algebra - True or False 1. – 5, – 5, – 5, – 5, – 5, … is an arithmetic sequence...
- math - The 1st,5th,13th term of an arithmetic sequence are the first 3 terms of ...
- math - in an arithmetic sequence the common difference is equal to 2.the first ...
- Can someone help me?! - The 1st, 5th and 13th terms of an arithmetic sequence ...
- Mathematics arithmetic sequence - The sum of second and sixth terms of an ...
- Math - The 13th termof an arithmetic sequence is 61 and the 20th term of the ...
- mathematics - 1.1 the third term of an arithmetic sequence is 8 and the 15 term ...

More Related Questions