Which answer describe the type of sequence? 3,9,15,21

The sequence is arithmetic

we know that

In an arithmetic sequence, the difference between consecutive terms is always the same and is called common difference

In this problem we have

3,9,15,21,...

Let

a1=3, a2=9,a3=15,a4=21

a4-a3=21-15=6

a3-a2=15-9=6

a2-a1=9-3=6

The sequence is arithmetic

The common difference is equal to

answer: the angel abc is 30°,and the abc is a triangle where the angel bac is 90°

so ac is the half of 24=12ft,and from the pythagorean theorem we find ba

ba=√bc^2-ac^2=√576-144=√432=12√3ft

so bac is 12+12√3=12(1+√3)ft