Question

...

Answer 1

oooohh... arithmetic sequence ^.^

Answer 2

Yes :<

Answer 3

first, see if you can find what's called the common difference d...
it's what you get if you subtract the first term from the second c'mon now... ^_^

Answer 4

And then? :))

Answer 5

and then..? what's d?

Answer 6

3

Answer 7

\[\Huge S_n=\frac{n}{2}[2a_1+(n-1)d]\]
N=Total terms
\(a_1\)=First term
d=Common difference here is 24-21=3
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We have to find the total number of terms, we can find that by using the last term, 72,
\[T_n=a_1+(n-1)(d)=72 \\ \\ 21+(n-1)(3)=72 \\ \\ n=18\]
Substitute in and find the sum

Answer 8

And now, you also have to find n, the number of terms in your sequence...
you can do that by solving this equation \[a_n = a_1+(n-1)d\] where a1 is the first term and a_n is the last.