You are Here: Home >< A-levels

# maths a level; sigma arithmetic problem watch

1. 30
∑ (4r+7)
r=3

Hello, yes i know it's quite late.

I'm very confused on how to solve this? i tried solving it the normal way i would if r=1, except by multiplying by 3 first. but the markscheme said it was wrong. (the answer should be 2044) how do i solve this? please explain.
2. Write out the first few terms to get the initial term and the difference term, then use the sum of an arithmetical series, which you can derive easily:

What values of a, d and n did you get?
3. (Original post by RogerOxon)
Write out the first few terms to get the initial term and the difference term, then use the sum of an arithmetical series, which you can derive easily:

What values of a, d and n did you get?
a = 19
d=4
n=30

is that correct??

i tried using that very same formula before, but to no avail.
4. (Original post by ZiggyStarDust_)
a = 19
d=4
n=30

is that correct??

However, I don't get the answer that you gave
5. (Original post by RogerOxon)

However, I don't get the answer that you gave
im worried that my markscheme might be wrong?? 2044 is apparently the answer.

So N is not 20? i don't understand what n could be then?
6. (Original post by ZiggyStarDust_)
30
∑ (4r+7)
r=3

Hello, yes i know it's quite late.

I'm very confused on how to solve this? i tried solving it the normal way i would if r=1, except by multiplying by 3 first. but the markscheme said it was wrong. (the answer should be 2044) how do i solve this? please explain.

because you'd have

can you see you'll be left with u_3 up to u_30 ?
7. r=3..30. If r were 1..30, you'd have 30 terms, so how many do you have for 3..30?
8. solve it like this https://gyazo.com/d401e7fef288e7176a9a7558275e8fdf
9. (Original post by ZiggyStarDust_)
im worried that my markscheme might be wrong?? 2044 is apparently the answer.
Sorry - I made a mistake - it is 2044.
10. (Original post by rogeroxon)
r=3..30. If r were 1..30, you'd have 30 terms, so how many do you have for 3..30?
o shet its 27 isnt it
11. (Original post by ZiggyStarDust_)
im worried that my markscheme might be wrong?? 2044 is apparently the answer.

So N is not 20? i don't understand what n could be then?
n=28 since the first term is 3 and the last term is 30 (so 28 terms all together).

Now solve, the answer should be 2044.
12. (Original post by tripplea)
n=28 since the first term is 3 and the last term is 30 (so 28 terms all together).

Now solve, the answer should be 2044.
why would it be 28 as a pose to 27?

because 30-3 is 27 right? so
13. (Original post by ZiggyStarDust_)
why would it be 28 as a pose to 27?

because 30-3 is 27 right? so
If
r=1-30 has 30 terms
then
r = 2-30 has 29 terms
r = 3-30 has 28 terms
14. (Original post by tripplea)
If
r=1-30 has 30 terms
then
r = 2-30 has 29 terms
r = 3-30 has 28 terms
thank you thank you thank you

so much

you've been very kind and helpful

its very much appreciated
you saved my arse

thank you
15. 30
∑ (4r+7)
r=3

19, 23, 27 - therefore the difference is 4
a=19, d=4, n=28

Sn = n/2[2a+(n-1)d]
28/2[2(19)+(28-1)4)
14[38+(27)4]
Sn = 2044

Hope that's clear enough for you
16. (Original post by ZiggyStarDust_)
thank you thank you thank you

so much

you've been very kind and helpful

its very much appreciated
you saved my arse

thank you
You're welcome. Do you get it now?

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 12, 2017
Today on TSR

And I hate it

### University open days

• Manchester Metropolitan University
Wed, 14 Nov '18
• University of Chester
Wed, 14 Nov '18
• Anglia Ruskin University
Ambitious, driven, developing your career & employability? Aspiring in your field, up-skilling after a career break? Then our Postgrad Open Evening is for you. Postgraduate
Wed, 14 Nov '18
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