# Thread: hard Sequence and Series Question!!

1. ## hard Sequence and Series Question!!

Its probably under harder 2unit in extension 1 but here's the question! please help

1,3,6,10,15....are called triangular numbers

Use the formula for the sum of an arithmetic series to find the 30th triangular number.  Reply With Quote

2. ## Re: hard Sequence and Series Question!! Originally Posted by kevda1st Its probably under harder 2unit in extension 1 but here's the question! please help

1,3,6,10,15....are called triangular numbers

Use the formula for the sum of an arithmetic series to find the 30th triangular number.
1 = 1
3 = 1+2
6 = 1 + 2 + 3
10 = 1 + 2 + 3 + 4

30th number will be

30/2 (2 + 29 x 1 ) = 465  Reply With Quote

3. ## Re: hard Sequence and Series Question!!

1, 3, 6,10,15,.......

New set of series you can make from top
1+2+3+4+5+6+......
true since 1+2 = T2 from original
1+2+3 = T3 from original
1+2+3+4 = T4 (" ")
so on....

a= 1 n=30 d= 1

s30= 30/2 (2x1+(30-1)x1)
= 15x31
= 465

lol kev, cya  Reply With Quote

4. ## Re: hard Sequence and Series Question!! Originally Posted by ghostman_on_3rd 1, 3, 6,10,15,.......

New set of series you can make from top
1+2+3+4+5+6+......
true since 1+2 = T2 from original
1+2+3 = T3 from original
1+2+3+4 = T4 (" ")
so on....

a= 1 n=30 d= 1

s30= 30/2 (2x1+(30-1)x1)
= 15x31
= 465

lol kev, cya
Sorry, I know this was a post a LOOONNGG time ago but why is d=1? Though you did say you made a replica series with 1+2+3+4+5... but may I ask of how exactly you derived that? If anyone passing this knows, please reach out. I'm sorry if it's a intuitive question, I just don't get the reasoning behind it yet. Thanks!   Reply With Quote

5. ## Re: hard Sequence and Series Question!! Originally Posted by orwellstudent Sorry, I know this was a post a LOOONNGG time ago but why is d=1? Though you did say you made a replica series with 1+2+3+4+5... but may I ask of how exactly you derived that? If anyone passing this knows, please reach out. I'm sorry if it's a intuitive question, I just don't get the reasoning behind it yet. Thanks! (1 layer) T1 = 1:

(2 layers) T2 = 3:

••

(3 layers) T3 = 6:

••
•••

(4 layers) T4 = 10:

••
•••
••••

(5 layers) T5 = 15:

••
•••
••••
•••••,  Reply With Quote

#### Thread Information

##### Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•