Find the sum of the first 70 terms of the sequence –6, –3, 0, 3, 6, a. 6824 b. 6825 c. 6826 d. 6827

The correct answer for the exercise shown above is the second option (Option b), which is:

 b. 6825

 The explanation is shown below:

 1. To solve this problem you must apply the following proccedure:

 2. You must apply this formula:

 Sn=(n/2)(2a1+(n-1)d)

 Where n=70, a1=-6 and d=3

 3. Then, when you substitute these values into the formula shown above, you obtain:

 Sn=(n/2)(2a1+(n-1)d)
 Sn=(70/2)(2(-6)+(70-1)d)
 Sn=(35)(295)

 4. Therefore, as you can see, the result is:

 Sn=6825