I don't understand this question! It's like simple math and I thought I got the answer but it's wrong and I don't understand why! Can someone show me how to answer this?

Find the sum of the first n terms of the sequence.
29, 33, 37, 41, ... ; n = 9

Here's the choices I have:

A) 423
B) 549/2
C) 405
D) 470

C 405

Step-by-step explanation:

a=29

c.d. d=33-29=4

n=9

This problem is an example of an arithmetic sequence. If you learned far, you will know that an arithmetic sequence is a sequence based on adding (or subtraction) and a geometric sequence is based on multiplication(or division)

Looking at this problem, we can clearly see that this is an arithmetic sequence because every value adds itself and four to get the next value.

The formula for arithmetic sequencing are as follows:

(n/2)(2a+(n-1)d)

Where n=number of terms, a=the first term, and d=difference between the second term and first term

(more simply, just see the attached picture for the equation+more hard version of the equation.)

We can see that it is easy to fill out the formula given our information above.

(9/2)(2(29)+(8)(4))     the bolded is the given.

To clarify, 9 is there in place of the n because there are 9 terms stated in the problem, 29 is in place of a because that is the first term, 8 is in place of n-1 because n which is 9, subtracted by 1 is 8, and 4 is in place of d because that is the difference from the second term to the first (33-29)

We can simplify that into

9/2(58+32)=9/2(90)=405

Which gives you C as your answer. Hope I helped!

(another way to solve this problem is to continue the pattern until 9 numbers arise

29, 33, 37, 41, 45, etc

and add them all up. But this isn't a good way to receive points and only should be used to check your work :) )