Arithmetic series

1. Dec 8, 2005

discombobulated

8a) The first and third terms of an arithmetic series are 3 and 27 respectively.
i) find the common difference
ii) find the sum of the first 11 terms of the series
b) find the sum of the inteers between 50 and 150 which are divisible by 8.
I have already done part a) and found the common difference to be 12 and the sum of the first 11 terms to be 693.
But i have no idea how to start part b, any suggestions please??

2. Dec 8, 2005

shmoe

Start making a list of the integers between 50 and 150 which are divisible by 8. Notice anything about this sequence?

3. Dec 8, 2005

HallsofIvy

Staff Emeritus
What is the first number, larger than or equal to 50, that is divisible by 8.
What is the last number, less than or equal to 150, that is divisible by 8?

How many numbers are the between 50 and 150 that are divisible by 8 (after you answer the first two questions, this is easy.)

Is there a formula for the sum? Hint: the title of this thread.