# Progression - Am I right?

• May 7th 2007, 12:40 PM
bobchiba
Progression - Am I right?
Hi, could someone tell me if I've done this correctly?

For the progression 1,4,7,10, .... state the
50th term and the sum of the first 50 terms.

I have 50th term as 148

I have sum of first 50 as 1475...

Cheers.
• May 7th 2007, 01:00 PM
CaptainBlack
Quote:

Originally Posted by bobchiba
Hi, could someone tell me if I've done this correctly?

For the progression 1,4,7,10, .... state the
50th term and the sum of the first 50 terms.

I have 50th term as 148

looks OK but if you told us how you found this it would be easier to
comment.

Quote:

I have sum of first 50 as 1475...
Sum of the first n terms of an AP with first term a and common difference d:

S(n) = (1/2) n (2a + (n-1)*d) = 3725

RonL
• May 7th 2007, 01:09 PM
bobchiba
regarding the sum,

I was shown in a previous thread a while ago that

Sn = n(a1 + an) / 2

so I ended up with 25( 1 + a20)
• May 7th 2007, 01:12 PM
bobchiba
sorry....just realised my mistake, should have been a50 not a20....it works out now,

thank you.
• May 7th 2007, 01:13 PM
CaptainBlack
Quote:

Originally Posted by bobchiba
regarding the sum,

I was shown in a previous thread a while ago that

Sn = n(a1 + an) / 2

so I ended up with 25( 1 + a20)

Why not 25(1+a50)?

Below is a brute force calculation of the sum:

Code:

``` > >n=0:49; > >a=1+3*n; > >sum(a)         3725 >```
RonL