Question

find the sum of the first 15 terms of the arithmetic sequence
-6,3,12,21

Answer 1

difference between each set:
+9, +9, +9... can you see the pattern now?

Answer 2

hello, @kamaal123
you can use the formula for sum of n terms of arithmetic series,
\(S_n = (n/2)(2a_1+(n-1)d)\)
where, a1 = first term = -6
n= 15 = number of terms
d= common difference = can you find it ???