You are Here: Home >< A-levels

# Sequences/series watch

given the 4th term of a sequence is 21 and the 7th term is 36 find the 10th term and the sum of the first 20 terms?
2. (Original post by jasminia)
given the 4th term of a sequence is 21 and the 7th term is 36 find the 10th term and the sum of the first 20 terms
Use the formula for arithmetic sequence for the 4th term and 7th term, solve simultaneously to find the value of the 1st term as well as the difference. Then just sub it in for the formula for the first 20th terms.
3. (Original post by kkboyk)
Use the formula for arithmetic sequence for the 4th term and 7th term, solve simultaneously to find the value of the 1st term as well as the difference. Then just sub it in for the formula for the first 20th terms.
but how do you find (or what is) the arithmetic sequence for the terms?
4. (Original post by jasminia)
but how do you find (or what is) the arithmetic sequence for the terms?
You use the formula started in your textbook and taught in lessons (assuming that you aren't a private candidate):
.

Here a is the starting point, and d is the difference between each term. We use as d is not used in the first term.
5. (Original post by kkboyk)
You use the formula started in your textbook and taught in lessons (assuming that you aren't a private candidate):
.

Here a is the starting point, and d is the difference between each term. We use as d is not used in the first term.
Thank you very much

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: January 23, 2017
Today on TSR

### Edexcel C2 Core Unofficial Markscheme!

Find out how you've done here

Poll

## All the essentials

### Student life: what to expect

What it's really like going to uni

### Essay expert

Learn to write like a pro with our ultimate essay guide.

### Create a study plan

Get your head around what you need to do and when with the study planner tool.

### Resources by subject

Everything from mind maps to class notes.

### Study tips from A* students

Students who got top grades in their A-levels share their secrets