Posted by **Brittany** on Wednesday, February 2, 2011 at 3:52pm.

The third and seventh terms of an arithmetic sequence are 10 and 20.

1) What is the value of the 1st term?

2) What is the value of the 9th term?

3) What is the equation to find the nth term in the sequence?

HELP!! PLEASE!!

- math -
**helper**, Wednesday, February 2, 2011 at 4:46pm
Arithmetic sequence

a, a + d, a + 2d, a + 3d, ...a +(n - 1)d

1)a3 = 10, a7 = 20

Since these are 4 places apart (7 - 3 = 4)

a7 = a3 + 4d

20 = 10 + 4d

10 = 4d

d = 10/4 = 2.5

So, the common difference, d = 2.5

To find the 1st term a,

a3 = a + 2d

10 = a + 2(2.5)

10 = a + 5

a = 5

2) to find 9th term

a9 = a + 8d

a9 = 5 + 8(2.5)

a9 = 5 + 20

a9 = 25

3) nth term

Last term = a + (n - 1)d

an = 5 + (n - 1)(2.5)

- math -
**switee**, Sunday, May 12, 2013 at 10:53am
4a2

## Answer This Question

## Related Questions

- Arithmetic Sequence - Given the third term of an arithmetic sequence less than ...
- Math - 1. Find the 12th term of the arithmetic sequence 2, 6, 10, … . 2. Solve ...
- Precalculus - 1. In an arithmetic sequence, the fifth term, u5, is greater than ...
- 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 - A sequence is formed by adding together the corresponding terms of a ...
- arithmetic - Determining the first three terms of an ArithmetiC progressionof ...
- Mathematics : Arithmetic Sequence - The 5th term and the 8th term of an ...
- math - The first and seventh terms of a sequence are both 8. Starting with the ...
- math - suppose an arithmetic sequence and a geometric sequence with common ...

More Related Questions